Anal Cancer - What is Anal Cancer? The Causes, Symptoms, Treatment, and Prevention

Anal cancer is cancer that develops in the anus, the opening of the rectum, which allows waste to pass from the large intestine to outside of the body. The anus is often confused with the rectum, which is the last several inches of the lower intestine. The rectum ultimately ends at the anus, where fecal material exits the body.
Like other parts of the body, the anus is susceptible to infections and diseases, such as anal cancer.

http://cancer.about.com/od/analcancer/a/anal_cancer.htm