Here's our new BustedHalo feature which explain different elements of Catholic teaching--like a glossary of sorts in our Googling God section which is scheduled for an upcoming facelift in about a week or so.
Catholics believe that Mary, the mother of Jesus, became pregnant not through a sexual encounter with a man, but by the power of the Holy Spirit. Joseph was betrothed to a woman who was pregnant and he knew he was not the father. He was troubled. God spoke to Joseph in a dream telling him, “that which is conceived in her is of the Holy Spirit.” (Matthew 1:20)
Many people, when they encounter this piece of Catholic tradition, have doubts that it could happen. Some even point to other pagan gods being born of virgins and suggest the possibility that Catholicism borrowed from these legends — especially since there is no mention of Mary’s virginity in Mark’s Gospel, the oldest of the four gospels.
Read the rest of the answer here and look for much more to come.