When you cut your hand or scrape your knee, it usually will stop bleeding on its own. That's because blood is usually able to form clots that protect you from bleeding too much.
But people with hemophilia have blood that doesn't clot the way it should. That means that they bruise and bleed easily and they may bleed a lot or for a long time after an injury. Hemophilia is a genetic condition (meaning it runs in families).