Some older games do not support wide screen resolutions and those games will look distorted on a wide screen (though they will usually still play just fine). Many older 3D games and (to my knowledge) all older 2D games are like this.
Some wide screen monitors support displaying 4:3 resolutions (like those of older games) on a wide screen without distortion (basically it scrunches it in on the screen with black areas on both sides). I have an LG Flatron W2252TQ (22" widescreen) that does this through it's on-screen menus.
Additionally, many video cards have a similar feature. I know that both of my laptops (one with an Intel video chipset and one with Nvidia) support this as does my desktop's Nvidia card. There's usually an option phrased "maintain aspect ratio for lower resolutions" or something to that effect in the video card's options.
Anyway, keep that in mind if it applies to you or any games you want play.