Anal masturbation is manual erotic stimulation

Masturbation

Masturbation refers to sexual stimulation of a person's own genitals, usually to the point of orgasm. The stimulation can be performed manually, by use of objects or tools, or by some combination of these methods. Masturbation is a common form of autoeroticism...

 focusing on the anus

Anus

The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...

 and rectum

Rectum

The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...

. For humans, common methods of anal masturbation include manual stimulation of the anal opening and the insertion of an object or objects such as fingers, feet, fists, phallic shaped items, water play, or sex toys such as anal beads

Anal beads

Anal beads are a sex toy consisting of multiple spheres or balls attached together in series which are continuously inserted through the anus into the rectum and then removed with varying speeds depending on the effect desired...

, butt plug

Butt plug

A butt plug is a sex toy designed to be inserted in the anus and rectum for sexual pleasure. In some ways, they are similar to a dildo, but they tend to be shorter, and must have a flanged end to prevent the device from being lost inside the rectum.- Basics :...

s, dildo

Dildo

A dildo is a sex toy, often explicitly phallic in appearance, intended for bodily penetration during masturbation or sex with partners.- Description and uses :...

s, vibrators

Vibrator (sex toy)

Vibrators are devices for the body and skin, to stimulate the nerves for a relaxing and pleasurable feeling. Some vibrators are designed to ergonomically stimulate erogenous zones for erotic stimulation.-History:...

 or specially designed prostate massagers.
Anal masturbation is practiced by men and women of all sexual orientation

Sexual orientation

Sexual orientation describes a pattern of emotional, romantic, or sexual attractions to the opposite sex, the same sex, both, or neither, and the genders that accompany them. By the convention of organized researchers, these attractions are subsumed under heterosexuality, homosexuality,...

s.
The most common explanation of how stimulation can be derived from anal masturbation is that the rectal opening contains many sensitive nerve endings.