Anisotropic, trilinear or bilinear

"Anisotropic filtering is a technique used to improve the sharpness of distant objects (removes fog). Enabling this option will enhance image quality but at the expense of a significant performance drop." pw33t ^^

In fact, when you look at distant objects, there is a kind of blurry film that covers them (this can be particularly seen in Counter-Strike). Activating anisotropic filtering will remove this blurry film, and the objects will be much sharper (as if you were right next to them).

So it’s better to choose anisotropic filtering if you want your game to look nicer, but if you’re looking for smoothness, it’s better to set it to trilinear or better yet, bilinear (but your game won't look particularly ugly either :)