A girl is a young woman, or a female child. If you have a 3-year-old daughter you have a little girl.
Girl comes from the Middle English garl, but no one knows where that word came from. It used to mean a child of either sex, but now it's only used for female children, paired with the word boy. A girl can be very young, or a young woman who's not quite out on her own in the world yet. Adults in relationships who aren't married call their female loves their girlfriend, even when the girl in question is really a woman.