A fistula is an abnormal communication between two epithelial surfaces. This can be anything from between two parts of the colon, between the bladder and the rectum (colo-vesical fistula), between the rectum and the vagina (recto-vaginal fistula), between the trachea and the esophagus (traheo-esophageal fistula) and so on and so forth.
Crohn's disease is famous for causing fistulas between two bits of colon, between colon and vagina/bladder and even colon and skin.