Anal fistula, or fistula-in-ano, is an abnormal connection between the epithelialised surface of the anal canal

Anal canal

The anal canal is the terminal part of the large intestine.It is situated between the rectum and anus, below the level of the pelvic diaphragm. It lies in the anal triangle of perineum in between the right and left ischioanal fossa....

 and (usually) the perianal skin.

Anal fistulae originate from the anal glands

Anal glands

The anal glands or anal sacs are small glands found near the anus in many mammals, including dogs and cats. They are not found in humans or other primates. They are paired sacs located on either side of the anus between the external and internal sphincter muscles. Sebaceous glands within the...

, which are located between the two layers of the anal sphincters and which drain into the anal canal

Anal canal

The anal canal is the terminal part of the large intestine.It is situated between the rectum and anus, below the level of the pelvic diaphragm. It lies in the anal triangle of perineum in between the right and left ischioanal fossa....

. If the outlet of these glands becomes blocked, an abscess

Abscess

An abscess is a collection of pus that has accumulated in a cavity formed by the tissue in which the pus resides due to an infectious process or other foreign materials...

 can form which can eventually point to the skin surface.