What sort of disorder is a fistula?

Abnormal connection. A fistula is when two tubes are connected that should not be. A fistula can be located almost anywhere in the body, but most commonly in the GI system.
Abnormal. A fistula is an abnormal communication between 2 organs. In the case of an anal fistula, between the rectum and the perianal skin. There are many other types of fistulas, however they all need to be addressed by an expert clinician to determine the propeer management.