Anus

"The anus is the end of the large intestine, below the rectum, through which stool (solid waste) leaves the body. The anus is formed partly from the outer skin layers of the body and partly from the intestine. Two ring-like muscles, called sphincter muscles, open and close the anal opening to let stool pass out of the body. The anal canal, the part of the anus between the rectum and the anal opening, is about 1½ inches long.
The skin around the outside of the anus is called the perianal area. Tumors in this area are skin tumors, not anal cancer."

Fistula

An anal fistula is a small channel that can develop between the end of the bowel and the skin near the anus. An anal fistula can cause bleeding and discharge when passing stools - and can be painful. An anal fistula can occur after surgery to drain an anal abscess. In some cases, an anal fistula causes persistent drainage. In other cases, where the outside of the channel opening closes, the result may be recurrent anal abscesses. The only cure for an anal fistula is surgery. Anal fistulae can present with many different symptoms:
1. Pain
2. Discharge — either bloody or purulent
3. Pruritus ani — itching
4. Systemic symptoms if abscess becomes infected.