Starting with Antialiasing. AA smooths out the jagged edges in games, simple as that. The more samples you use (2x 4x 8x) the more work it its for the GPU.
This screenshot comparison I made shows the difference between no AA at all, and the max Skyrim setting of 8x samples. Note that the only difference the AA makes is the jaggedness of edges
This chart shows the FPS change between the different AA settings on different resolutions (from the Nvidia article)
|almost 20 FPS difference|
Now for Anisotropic Filtering. Anisotropic Filtering improves the clarity of textures, making a noticeable and fantastic difference. This picture comparison is from the Nvidia article, I cut it together and added the thoughtful comparison emotional response indicator.
|The difference between no AF and 16x AF|