God sent the angel Gabriel to Nazareth, to the virgin Mary. And the angel said, "Rejoice, highly favored one, the Lord is with you; blessed are you among women!" She was afraid, and the angel said, "Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bring forth a Son, and shall call His name Jesus..." And Mary didn't know how this could be possible, but the angel assured her, "The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore, also, that the Holy One who is to be born will be called the Son of God..." Then, Mary said, "Behold the handmaid of the Lord! Let it be to me according to your word." (my paraphrase of Luke 1:26-38)
I love the Annunciation because of Mary's courage and faith. She was afraid, but she was willing to trust in God's will for her. And because she was willing to trust, the rest of us received the greatest gifts: the Incarnation and the Resurrection of Christ.
Finally, below is a youtube video of a Catholic song, based on the Haily Mary prayer, that I just love.