Fistula is a medical term that describes where a 'tunnel' forms somewhere inside the body. In the case of an anal fistula it is a tunnel connecting the lining of the anus and the skin. An opening on the surface of the skin forms and blood or pus may start to discharge from it. Occasionally a small, uncomfortable lump may also appear.
A fistula can simply appear, with no apparent underlying cause, however a fistula often occurs after an anal abscess has been drained. Crohn's diseases does increase the likelihood of developing a fistula, however the majority of people who develop a fistula have no underlying bowel conditions.
Apart from the discharge of blood/pus and the (occasional) formation of an associated lump as described above, there can often be very few symptoms, such as some discomfort, and these symptoms can be intermittent.
A fistula will only very rarely clear up on its own - in most cases surgery is needed.