The Annunciation, an enunciation...

Who was Gabriel? Why was he sent?

Who was Zacharias - a Priest of Abijah

"Betroth: To promise "by one's truth." Men and women were betrothed when they were engaged to be married. This usually took place a year or more before marriage. From the time of betrothal the woman was regarded as the lawful wife of the man to whom she was betrothed (Deuteronomy 28: 30; Judges 14: 2, 8; Matthew 1: 18-21). The term is figuratively employed of the spiritual connection between God and his people (Hosea 2: 19, 20)."

How old would Mary have been?
In the Jewish culture of New Testament times, a young woman became marriageable at age twelve and a half. At a ceremony of betrothal, the bride and groom would exchange marital consent, but normally the bride would remain in the house of her father for somewhere between three months and a year. Marriage had the aspect of a man acquiring title to his bride, and only later did he acquire actual possession.

Once betrothed a couple was considered legally married, even though the marriage was not yet consummated. 

"The groom then poured a cup of wine for the prospective bride. Because Jewish law stated that a woman could not be forced to marry a man distasteful to her, the bride was ultimately allowed to choose whether to accept or reject the groom’s proposal. If she drank the cup he offered, they were betrothed. The groom would formally accept his bride with another ritual statement, often “Thou art set apart (or consecrated) for me according to the law of Moses and Israel.” Interestingly, the same word for “set apart” was also used to describe a dedicated temple; the bride was considered a temple now set apart for her husband. From this point on, the bride would wear a veil over her hair in public to indicate her status as a betrothed or married woman."
Historical and Cultural background of Jewish Marriage

We know that Mary and Joseph had completed the contract of betrothal from the testimony given by St. Matthew (1: 18) and St. Luke (2: 5).  What causes some confusion, though is Luke's reference to "Mary his espoused wife" when Mary and Joseph were already in Bethlehem, seeking a place to give birth to our Infant Lord. For them to be in Jerusalem together, Joseph had obviously taken Mary from her father's house, and it seems clear that their betrothal had already been converted into a marriage. As he was aware of the circumstances of our Lord's conception, Luke was probably following the custom of referring to a non-consummated marriage as an "espousal."

Could Mary have been killed for becoming pregnant?