BoogleTech has up a "How-To" guide on using RivaTuner covering almost every aspect of it in a fair bit of detail.
RivaTuner is a utility that allows you to change settings not normally available to you in Display Properties. It also gives you access to unofficial settings as well, such as Anisotropic Filtering in Direct3D (normally only available in OpenGL). RivaTuner has a lot of features, hence the reason for this guide, so you know what they do, and whether you want to use any of these features. One final note, RivaTuner only supports nVidia graphics cards.More information at BoogleTech