The anus is the external opening of the anal canal.
If you peek between your buns with a mirror you’ll see soft, pink folds of tissue, looking a little bit wrinkly.
Everybody has some hair growing around the anus. These may be fine and light coloured, so that you barely notice them when you look. But they can also be dark and coarse. With both guys and girls they can spread into the crack.
Because the anus is very sensitive many stimulate the anus during masturbation or love making.

If I insert a finger in my girlfriend’s anus, will it get dirty?

See also: the anatomy of the anal muscles, the shape of the rectum and anal sex.