A girl's hymen is a very thin layer of skin at the opening of their vagina which is there when girls are born. It has a small hole in it which allows for blood to come out when girls start menstruating. If you are a "virgin" the hymen is still there, but after you have sex then your hymen gets "torn" and is no longer there. The first time people have sex when the hymen gets completely torn it can cause some bleeding and there may be a bit of soreness. This varies greatly from person to person and depends, to a certain extent, on how thick your hymen is. However having "sex" is not the only way that your hymen can get torn. It may very occasionally happen when riding bicycles, riding horses. It gets torn a bit if you use tampons, but even if this happens you are still technically a "virgin".