A bruise is an injury that is characterized by skin discoloration due to bleeding under the skin. The bruise is usually black or blue. A bruise could be caused by hitting something, rigorous exercise, or thinning skin. Bruises are usually treated with cold compresses, elevation, rest, and pain relievers. Follow the links below to find WebMD's comprehensive coverage about how a bruise is caused, what it looks like, how to treat it, and much more.