Uterine bleeding means that a girl looses blood in between two periods.
This is quite common when a girl starts using oral contraceptives (see: spotting). It’s also quite normal when girls start having periods, blood loss being irregular at first. Sometimes it takes one or two years before a regular pattern is established.
If something enters your vagina for the first time (fingers, a tampon, a dildo, penis or other object) the piece of tissue around the vaginal opening may tear, causing some bleeding.
Abnormal bleeding can occur when you have a STD. With chlamydia and gonorrhoea for instance girls may bleed after intercourse. So If you have had intercourse without using condoms you should have it checked out.
Blood loss can also occur when you have a miscarriage or ectopic pregnancy.

Uterine bleeding can be a sign of something harmless, like a polyp, but also be a symptom of a serious disease, like cervical cancer. If you have uterine bleeding (that cannot be explained by periods that just started, by short usage of the pill or by a torn hymen) have it checked out as soon as possible.

See also: spotting