Any sex involving the anus. There are three degrees to butt sex:
1. Regular butt sex. That is anal sex involving plenty of lubrication and great care to avoid injury.
2. Rough butt sex. Anal sex involving little lube by those who like it rough. Spit or similar substances may also be used.
3. Angry butt sex. No lube, no preparation. Obviously not a very happy time for the recipient.
1. The kind of butt sex most people have.
2. Think Brokeback Mountain.
3. ...ummm....I don't think this actually happens.