The anus is the final opening of our rectum and its primary (fundamental!) function is to expel faeces and gas. This areas is not only full of nerves and elastic muscles but also has two sphincters, and inner involuntary one and an external one. During anal sex the penetration of this area could be painful if not done slowly and with the help of lube. That's because this sphincter is very narrow at the end (or entrance) - not so much once you have overcame that step. Nerves in this area can be stimulated in this way and give you pleasure. The anus is different from the vagina when it comes to lubrication. It's normally a little bit lubricated but that's not enough for a non painful penetration, and it doesn't produce extra fluids as the vagina does when aroused. The use of a lubricant is necessary to make it a pleasurable experience! Beside anal play with fingers, toys or penis, there is also anilingus -known also as rimming- the licking of the anus area.