Anal sex is the penetration of a person's rectum by a penis.
Anal sex is generally restricted to the insertion of an erect penis into the rectum. It does not 'generally' refer to the insertion of other objects, though in the 'gay' side of anal sex, various forms of ass play can involve other objects, before and after, anal sex.
While there might be some who simply engage in anal sex, fact is that it is usually a culmination of previous sexual acts, and that anal sex is the final stage of intercourse among gay males.
Unlike its depiction in porn movies, most anal sex involves a great deal of lubrication prior to insertion. It is also considered a highly risky sexual act, due to the HIV/AIDS disease.
[heterosexuals] while it is estimated that on average heterosexual couples engage in anal sex, it's popularity is limited to between 30 and 40%, depending on the study. Straight males claim it is more intense due the tightness of the woman's rectal muscles.
[homosexuals] As surprising as it is, not all gay men engage in anal sex. Some find it uncomfortable, as many women do, or they refuse due to the increased risks, which aren't just limited to the transmission of diseases. There are many different 'positions' for anal sex, some of which require a bit of practice to perfect. Generally there is the missionary position, or on the belly, of side by side. Variations exist, and are based on each person's abilities.
Anal Sex is about the stimulation of the male prostate gland, located close to the rectum. This can also be achieved through other means than simply inserting a penis or fingers. While some who refuse anal sex, they are fans of other gay sex acts involving the rectum, such as rimming.