You have 3 options via the on-screen-display. I'm not 100% with my definitions, they are simply what I'm inferring from screwing around with the monitor for a while.
1to1: I believe this displays the image pixel for pixel and the image will be centered in the display with large black bars on the top, bottom, and sides.
Full: This will stretch the image so that it takes up the full screen and will cause the picture to look wider than normal.
Aspect: This will will expand the image, keeping the 4:3 aspect ratio, until it reaches one of the edges of the screen. For example, when hooking up my gamecube via composite input, this expands the image until it is touching the top and bottom of the screen, leaving black bars on the sides with the picture in the correct proportions.
P.S. I'm at work right now, so I'm not in front of the monitor to double check my thoughts. Everybody, please correct me if I'm wrong.