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."

Proctitis

Proctitis is defined as inflammation of the anus (the opening) and lining of the rectum (lower part of the intestine leading to the anus). Symptoms of proctitis can vary greatly. One may at first have only minor problems. Proctitis affects the last 6 inches of the rectum and can cause the following:
1. Pain during a bowel movement
2. Soreness in the anal and rectal area
3. Feeling that you didn't completely empty the bowels after a bowel movement
4. Involuntary spasms and cramping during bowel movements
5. Bleeding, and possibly a discharge
Proctitis can last a short time or become a chronic condition (last for weeks or months or longer). Proctitis has many causes, but sexually transmitted diseases (STDs) are the most common. Gonorrhea, syphilis, herpes, anal warts, and chlamydia are the most common cause of sexually transmitted proctitis. Proctitis is increasingly more common in homosexual men and in people engaging in oral-anal or anal intercourse with many partners.