Open wounds in the skin. Usually gaping and the source of a nasty odor, they usually occur on or around the anus and buttox areas. See George W Olson for an example.
The breaking of the skin around and inside the anus, generally caused by the skin being stretched too far. Some bleeding may occur as a result.
Anal fissure is typically caused by sexual acts which involve the insertion of anything large into the anus. Anal sex alone can cause anal fissure, though this is highly unlikely if enough lubrication is used (and, if the penetrating partner is careful).
It is more likely to occur during sexual acts such as fisting.