http://www.extremetech.com/article2/0,1558,1764462,00.asp
Note: Not all games scale well to widescreen mode. You can find a wealth of information about gaming on widescreen displays at the Wide Screen Gaming Forum. If you're forced to play at 4:3 resolution, you can choose one of three modes: fill (stretched), Aspect Ratio, or 1:1. The 1:1 mode uses only the number of pixels actually requested. Aspect Ratio preserves the requested aspect ratio. For example, if you run at 1024x768, you'll get a 4:3 display centered in the screen, with black pillar bars on either side. Fill mode stretches the image from 4:3 mode to full screen mode, distorting the picture in the process. Note that you cannot choose which mode you're in when you're running at the native resolution. You have to switch to a different (lesser) resolution to change the modeone of the odd quirks of the on-screen display.