Native resolution: Because an LCD uses a matrix of pixels to display its image, it has a fixed (or native) resolution at which the display looks best. A 15in LCD has a native resolution of 1024x768, while most 17in, 18in and 19in models use a 1280x1024 native resolution. Widescreen 23in and 24in models usually have a native resolution of 1920x1200, and 30in widescreens have a resolution of 2560x1600. If you are using Windows XP or earlier and set the monitor to a lower-than-native resolution - to make the icons and text on your screen look bigger, for instance - the image will almost certainly be less defined, because the display will use only a portion of the pixels it contains and will scale up the resulting image to fill the screen. Keep in mind that you can never exceed the native resolution of an LCD monitor. So, for example, you will not be able to display 1600x1200 resolution on an LCD with a native resolution of 1280x1024.
Though scaling technologies have improved in recent years, you're still likely to be disappointed with their results. On Windows XP and earlier, text most readily shows pronounced jaggedness at non-native resolutions. Windows Vista may reduce this effect with its vector graphics, but it's still advisable to use a monitor at its native resolution. So a particular LCD is a good choice if its native resolution is one you are comfortable using for all applications.
Aspect ratio: Most LCDs have a screen aspect ratio of about 3:4, much like a regular format TV. However, widescreen monitors have an aspect ratio closer to the 16:10. The widescreen format becomes useful for working in large spreadsheets or in programs that contain many toolbars or palettes. Surfing the Web or working in multiple documents can be more enjoyable as you can put two good-sized windows side-by-side on the same screen. It's appealing for watching DVDs as well, although the image quality may not be as good as on a TV. Many users see a widescreen monitor as an upgrade from using two smaller monitors. A dual-monitor setup is usually the less expensive proposition.
One important thing to keep in mind: The screen size is measured diagonally, and the area of a widescreen monitor's display is smaller than that of a regular format display of the same size. In other words, a 21in widescreen monitor shows about as many pixels as you'd expect from a regular format 19in monitor. In the days of CRT monitors, vendors would state a tube size, say 21in, but the diagonal of the viewable screen would be from one to two inches less. With LCDs, the stated diagonal is always the true measurement from the viewable screen.