The Blessed Virgin Mary is the mother of Jesus Christ, the mother of God.
(Taken from the Catholic Encyclopedia)
In general, the theology and history of Mary the Mother of God follow the chronological order of their respective sources, i.e. the Old Testament, the New Testament, the early Christian and Jewish witnesses.
Mary prophesied in the Old Testament
The Old Testament refers to Our Blessed Lady both in its prophecies and its types or figures.
The first prophecy referring to Mary is found in the very opening chapters of the Book of Genesis (3:15): "I will put enmities between thee and the woman, and thy seed and her seed; she shall crush thy head, and thou shalt lie in wait for her heel." This rendering appears to differ in two respects from the original Hebrew text:
(1) First, the Hebrew text employs the same verb for the two renderings "she shall crush" and "thou shalt lie in wait"; the Septuagint renders the verb both times by terein, to lie in wait; Aquila, Symmachus, the Syriac and the Samaritan translators, interpret the Hebrew verb by expressions which mean to crush, to bruise; the Itala renders the terein employed in the Septuagint by the Latin "servare", to guard; St. Jerome  maintains that the Hebrew verb has the meaning of "crushing" or "bruising" rather than of "lying in wait", "guarding". Still in his own work, which became the Latin Vulgate, the saint employs the verb "to crush" (conterere) in the first place, and "to lie in wait" (insidiari) in the second. Hence the punishment inflicted on the serpent and the serpent's retaliation are expressed by the same verb: but the wound of the serpent is mortal, since it affects his head, while the wound inflicted by the serpent is not mortal, being inflicted on the heel.
(2) The second point of difference between the Hebrew text and our version concerns the agent who is to inflict the mortal wound on the serpent: our version agrees with the present Vulgate text in reading "she" (ipsa) which refers to the woman, while the Hebrew text reads hu' (autos, ipse) which refers to the seed of the woman. According to our version, and the Vulgate reading, the woman herself will win the victory; according to the Hebrew text, she will be victorious through her seed. In this sense does the Bull "Ineffabilis" ascribe the victory to Our Blessed Lady. The reading "she" (ipsa) is neither an intentional corruption of the original text, nor is it an accidental error; it is rather an explanatory version expressing explicitly the fact of Our Lady's part in the victory over the serpent, which is contained implicitly in the Hebrew original. The strength of the Christian tradition as to Mary's share in this victory may be inferred from the retention of "she" in St. Jerome's version in spite of his acquaintance with the original text and with the reading "he" (ipse) in the old Latin version.
As it is quite commonly admitted that the Divine judgment is directed not so much against the serpent as against the originator of sin, the seed of the serpent denotes the followers of the serpent, the "brood of vipers", the "generation of vipers", those whose father is the Devil, the children of evil, imitando, non nascendo (Augustine).  One may be tempted to understand the seed of the woman in a similar collective sense, embracing all who are born of God. But seed not only may denote a particular person, but has such a meaning usually, if the context allows it. St. Paul (Galatians 3:16) gives this explanation of the word "seed" as it occurs in the patriarchal promises: "To Abraham were the promises made and to his seed. He saith not, and to his seeds, as of many; but as of one, and to his seed, which is Christ". Finally the expression "the woman" in the clause "I will put enmities between thee and the woman" is a literal version of the Hebrew text. The Hebrew Grammar of Gesenius-Kautzsch  establishes the rule: Peculiar to the Hebrew is the use of the article in order to indicate a person or thing, not yet known and not yet to be more clearly described, either as present or as to be taken into account under the contextual conditions. Since our indefinite article serves this purpose, we may translate: "I will put enmities between you and a woman". Hence the prophecy promises a woman, Our Blessed Lady, who will be the enemy of the serpent to a marked degree; besides, the same woman will be victorious over the Devil, at least through her offspring. The completeness of the victory is emphasized by the contextual phrase "earth shall thou eat", which is according to Winckler  a common old-oriental expression denoting the deepest humiliation .
The second prophecy referring to Mary is found in Isaias 7:1-17. Critics have endeavoured to represent this passage as a combination of occurrences and sayings from the life of the prophet written down by an unknown hand . The credibility of the contents is not necessarily affected by this theory, since prophetic traditions may be recorded by any writer without losing their credibility. But even Duhm considers the theory as an apparent attempt on the part of the critics to find out what the readers are willing to bear patiently; he believes it is a real misfortune for criticism itself that it has found a mere compilation in a passage which so graphically describes the birth-hour of faith.
According to 2 Kings 16:1-4, and 2 Chronicles 27:1-8, Achaz, who began his reign 736 B.C., openly professed idolatry, so that God gave him into the hands of the kings of Syria and Israel. It appears that an alliance had been concluded between Phacee, King of Israel, and Rasin, King of Damascus, for the purpose of opposing a barrier to the Assyrian aggressions. Achaz, who cherished Assyrian proclivities, did not join the coalition; the allies invaded his territory, intending to substitute for Achaz a more subservient ruler, a certain son of Tabeel. While Rasin was occupied in reconquering the maritime city Elath, Phacee alone proceeded against Juda, "but they could not prevail". After Elath had fallen, Rasin joined his forces with those of Phacee; "Syria hath rested upon Ephraim", whereupon "his (Achaz') heart was moved, and the heart of his people, as the trees of the woods are moved with the wind". Immediate preparations must be made for a protracted siege, and Achaz is busily engaged near the upper pool from which the city received the greater part of its water supply. Hence the Lord says to Isaias: "Go forth to meet Achaz. . .at the end of the conduit of the upper pool". The prophet's commission is of an extremely consoling nature: "See thou be quiet; hear not, and let not thy heart be afraid of the two tails of these firebrands". The scheme of the enemies shall not succeed: "it shall not stand, and this shall not be." What is to be the particular fate of the enemies?
- Syria will gain nothing, it will remain as it has been in the past: "the head of Syria is Damascus, and the head of Damascus is Rasin".
- Ephraim too will remain in the immediate future as it has been hitherto: "the head of Ephraim is Samaria, and the head of Samaria the son of Romelia"; but after sixty-five years it will be destroyed, "within threescore and five years Ephraim shall cease to be a people".
Achaz had abandoned the Lord for Moloch, and put his trust in an alliance with Assyria; hence the conditional prophecy concerning Juda, "if you will not believe, you shall not continue". The test of belief follows immediately: "ask thee a sign of the Lord thy God, either unto the depth of hell or unto the height above". Achaz hypocritically answers: "I will not ask, and I will not tempt the Lord", thus refusing to express his belief in God, and preferring his Assyrian policy. The king prefers Assyria to God, and Assyria will come: "the Lord shall bring upon thee and upon thy people, and upon the house of thy father, days that have not come since the time of the separation of Ephraim from Juda with the king of the Assyrians." The house of David has been grievous not merely to men, but to God also by its unbelief; hence it "shall not continue", and, by an irony of Divine punishment, it will be destroyed by those very men whom it preferred to God.
Still the general Messianic promises made to the house of David cannot be frustrated: "The Lord Himself shall give you a sign. Behold a virgin shall conceive, and bear a son, and his name shall be called Emmanuel. He shall eat butter and honey, that he may know to refuse the evil and to choose the good. For before the child know to refuse the evil, and to choose the good, the land which thou abhorrest shall be forsaken of the face of her two kings." Without answering a number of questions connected with the explanation of the prophecy, we must confine ourselves here to the bare proof that the virgin mentioned by the prophet is Mary the Mother of Christ. The argument is based on the premises that the prophet's virgin is the mother of Emmanuel, and that Emmanuel is Christ. The relation of the virgin to Emmanuel is clearly expressed in the inspired words; the same indicate also the identity of Emmanuel with the Christ.
The connection of Emmanuel with the extraordinary Divine sign which was to be given to Achaz predisposes one to see in the child more than a common boy. In 8:8, the prophet ascribes to him the ownership of the land of Juda: "the stretching out of his wings shall fill the breadth of thy land, O Emmanuel". In 9:6, the government of the house of David is said to be upon his shoulders, and he is described as being endowed with more than human qualities: "a child is born to us, and a son is given to us, and the government is upon his shoulders, and his name shall be called Wonderful, Counsellor, God the Mighty, the Father of the World to Come, and the Prince of Peace". Finally, the prophet calls Emmanuel "a rod out of the root of Jesse" endowed with "the spirit of the Lord. . .the spirit of wisdom and of understanding, the spirit of counsel, and of fortitude, the spirit of knowledge and of godliness"; his advent shall be followed by the general signs of the Messianic era, and the remnant of the chosen people shall be again the people of God (11:1-16).
Whatever obscurity or ambiguity there may be in the prophetic text itself is removed by St. Matthew (1:18-25). After narrating the doubt of St. Joseph and the angel's assurance, "that which is conceived in her is of the Holy Ghost", the Evangelist proceeds: "now all this was done that it might be fulfilled which the Lord spoke by the prophet, saying: Behold a virgin shall be with child, and bring forth a son, and they shall call his name Emmanuel." We need not repeat the exposition of the passage given by Catholic commentators who answer the exceptions raised against the obvious meaning of the Evangelist. We may infer from all this that Mary is mentioned in the prophecy of Isaias as mother of Jesus Christ; in the light of St. Matthew's reference to the prophecy, we may add that the prophecy predicted also Mary's virginity untarnished by the conception of the Emmanuel .
A third prophecy referring to Our Blessed Lady is contained in Micah 5:2-3: "And thou, Bethlehem, Ephrata, art a little one among the thousands of Juda: out of thee shall be come forth unto me that is to be the ruler in Israel, and his going forth is from the beginning, from the days of eternity. Therefore will he give them up till the time wherein she that travaileth shall bring forth, and the remnant of his brethren shall be converted to the children of Israel." Though the prophet (about 750-660 B.C.) was a contemporary of Isaias, his prophetic activity began a little later and ended a little earlier than that of Isaias. There can be no doubt that the Jews regarded the foregoing prediction as referring to the Messias. According to St. Matthew (2:6) the chief priests and scribes, when asked where the Messias was to be born, answered Herod in the words of the prophecy, "And thou Bethlehem the land of Juda. . ." According to St. John (7:42), the Jewish populace gathered at Jerusalem for the celebration of the feast asked the rhetorical question: "Doth not the Scripture say that Christ cometh of the seed of David, and from Bethlehem, the town where David was?" The Chaldee paraphrase of Micah 5:2, confirms the same view: "Out of thee shall come forth unto me the Messias, that he may exercise dominion in Israel". The very words of the prophecy admit of hardly any other explanation; for "his going forth is from the beginning, from the days of eternity".
But how does the prophecy refer to the Virgin Mary? Our Blessed Lady is denoted by the phrase, "till the time wherein she that travaileth shall bring forth". It is true that "she that travaileth" has been referred to the Church (St. Jerome, Theodoret), or to the collection of the Gentiles united with Christ (Ribera, Mariana), or again to Babylon (Calmet); but, on the one hand, there is hardly a sufficient connection between any of these events and the promised redeemer, on the other hand, the passage ought to read "till the time wherein she that is barren shall bring forth" if any of these events were referred to by the prophet. Nor can "she that travaileth" be referred to Sion: Sion is spoken of without figure before and after the present passage so that we cannot expect the prophet to lapse suddenly into figurative language. Moreover, the prophecy thus explained would not give a satisfactory sense. The contextual phrases "the ruler in Israel", "his going forth", which in Hebrew implies birth, and "his brethren" denote an individual, not a nation; hence we infer that the bringing forth must refer to the same person. It has been shown that the person of the ruler is the Messias; hence "she that travaileth" must denote the mother of Christ, or Our Blessed Lady. Thus explained the whole passage becomes clear: the Messias must be born in Bethlehem, an insignificant village in Juda: his family must be reduced to poverty and obscurity before the time of his birth; as this cannot happen if the theocracy remains intact, if David's house continues to flourish, "therefore will he give them up till the time wherein she that travaileth shall bring forth" the Messias. 
A fourth prophecy referring to Mary is found in Jeremias 31:22; "The Lord has created a new thing upon the earth: A woman shall compass a man". The text of the prophet Jeremias offers no small difficulties for the scientific interpreter; we shall follow the Vulgate version of the Hebrew original. But even this rendering has been explained in several different ways: Rosenmuller and several conservative Protestant interpreters defend the meaning, "a woman shall protect a man"; but such a motive would hardly induce the men of Israel to return to God. The explanation "a woman shall seek a man" hardly agrees with the text; besides, such an inversion of the natural order is presented in Isaias 4:1, as a sign of the greatest calamity. Ewald's rendering, "a woman shall change into a man", is hardly faithful to the original text. Other commentators see in the woman a type of the Synagogue or of the Church, in man the type of God, so that they explain the prophecy as meaning, "God will dwell again in the midst of the Synagogue (of the people of Israel)" or "the Church will protect the earth with its valiant men". But the Hebrew text hardly suggests such a meaning; besides, such an explanation renders the passage tautological: "Israel shall return to its God, for Israel will love its God". Some recent writers render the Hebrew original: "God creates a new thing upon the earth: the woman (wife) returns to the man (her husband)". According to the old law (Deuteronomy 24:1-4; Jeremiah 3:1) the husband could not take back the wife once repudiated by him; but the Lord will do something new by allowing the faithless wife, i.e. the guilty nation, to return to the friendship of God. This explanation rests upon a conjectural correction of the text; besides, it does not necessarily bear the Messianic meaning which we expect in the passage.
The Greek Fathers generally follow the Septuagint version, "The Lord has created salvation in a new plantation, men shall go about in safety"; but St. Athanasius twice  combines Aquila's version "God has created a new thing in woman" with that of the Septuagint, saying that the new plantation is Jesus Christ, and that the new thing created in woman is the body of the Lord, conceived within the virgin without the co-operation of man. St. Jerome too  understands the prophetic text of the virgin conceiving the Messias. This meaning of the passage satisfies the text and the context. As the Word Incarnate possessed from the first moment of His conception all His perfections excepting those connected with His bodily development, His mother is rightly said to "compass a man". No need to point out that such a condition of a newly conceived child is rightly called "a new thing upon earth". The context of the prophecy describes after a short general introduction (30:1-3) Israel's future freedom and restoration in four stanzas: 30:4-11, 12-22; 30:23; 31:14, 15-26; the first three stanzas end with the hope of the Messianic time. The fourth stanza, too, must be expected to have a similar ending. Moreover, the prophecy of Jeremias, uttered about 589 B.C. and understood in the sense just explained, agrees with the contemporary Messianic expectations based on Isaias 7:14; 9:6; Micah 5:3. According to Jeremias, the mother of Christ is to differ from other mothers in this, that her child, even while within her womb, shall possess all those properties which constitute real manhood . The Old Testament refers indirectly to Mary in those prophecies which predict the Incarnation of the Word of God.
(Taken from the Catholic Encyclopedia)