SmoothType makes your screen look more like the printed page. Using a technique called anti-aliasing, SmoothType blurs the jagged edges of bitmap fonts with shades of gray, effectively doubling your screen resolution. This is the similar to the technique used by some laser printers of varying the size of the dots to improve the printed image quality. This is also the reason why a television picture can look so crisp even though TV resolution is really quite low.
Version 2.3.1 fixes a problem drawing smoothed text after switching accounts under Multiple Users. Version 2.3 introduced subpixel smoothing optimized for LCD (flat panel) screens.