Foul-smelling stools are stools with a very bad odor. They usually have to do with what you eat, but may be a sign of a medical condition.
Stools normally have an unpleasant odor, but one that is recognized as fairly common. Stools that have an extremely bad, unusual odor may be due to certain medical conditions. Foul-smelling stools also have normal causes, such as diet changes.