An inflamatory track which has an external opening in the perianal skin and an internal opening in the anal canal or rectum.This track is lined by unhealthy granulation tissue and fibrous tissue.This can be start by a boil or abscess from perianal area, after burst continious pus / liquid discharge from that area and pain has subsided after the pus has discharged out however after a certain interval of time; the swelling and pain may reappear which is again followed by pus discharge and subsequent relief in the symptoms. This cycle goes on. 

How ever in other cases regular discharges & stickiness in the anal area may be noted. Even today when the modern surgery is at its peek level, fistula in ano is still a challenge to the surgeons for its incidence of recurrence after surgery & post-operative complications like stool incontinence, pain and bleeding etc.

Causes:

Anal gland infection with formation of boil or abscess at perianal region which bursts with discharge of pus / blood. Patients with pulmonary TB have got 1-2 % chances of developing fistula in ano with watery discharge without pus. Recurrent fistula may be associated with Diabetes mellitus, Chronic Ulcerative colitis, Prostate infection.