Anisotropic filtering : the highest quality filtering available in current consumer 3D graphics cards. Simpler, “isotropic” techniques use only square mipmaps which are then interpolated using bi– or trilinear filtering. (Isotropic means same in all directions, and hence is used to describe a system in which all the maps are squares rather than rectangles or other quadrilaterals.)
When a surface is at a high angle relative to the camera, the fill area for a texture will not be approximately square. Consider the common case of a floor in a game: the fill area is far wider than it is tall. In this case, none of the square maps are a good fit. The result is blurriness and/or shimmering, depending on how the fit is chosen. Anisotropic filtering corrects this by sampling the texture as a non-square shape. Some implementations simply use rectangles instead of squares, which are a much better fit than the original square and offer a good approximation.
However, going back to the example of the floor, the fill area is not just compressed vertically, there are also more pixels across the near edge than the far edge. Consequently, more advanced implementations will use trapezoidal maps for an even better approximation (at the expense of greater processing).
In either rectangular or trapezoidal implementations, the filtering produces a map, which is then bi– or trilinearly filtered, using the same filtering algorithms used to filter the square maps of traditional mipmapping.