Viewing angle: Measured in degrees, an LCD's viewing angle indicates how far you can move to the side (or above, or below) from the centre of the display before the image quality deteriorates to unacceptable levels. No matter what sized monitor you use, a wide viewing angle becomes increasingly important the more you care about getting accurate, consistent colours for design work or for tweaking digital photos. Each vendor determines its own criteria for this, as no industry-standard method has been established for measuring viewing angle. As a result, the numbers may not be comparable from one vendor to another, but they can indicate relative performance among models from the same company.
The best way to judge viewing angle is to see the monitor for yourself, but you can eliminate some models from consideration if even their vendor-reported viewing angles are below a certain value. The larger the monitor, the more important a wide viewing angle is. On monitors measuring more than 17in, a viewing angle of at least 150 degrees is advisable, since the edge of the screen is at a greater angle to someone sitting directly in front of it. Additionally, people are more likely to be able to share a large monitor when working or giving a group presentation.
The choice of panel technology affects the viewable angle. Some LCDs use twisted nematic (TN - the most common type of liquid crystal being used) panels, which have small viewing angles. On a TN screen, brightness drops and colours change as you move to the side or up and down. This can mean that your work will look different if you adjust your chair or your posture. It also makes it difficult to share your work with someone who sits next to you (although additional hardware, like a physical swivel adjustment, can help with this). A few years ago, TN panels appeared to be on their way out, but recent interest in TN's fast pixel-response times has brought more new models to the market. Other panel types, such as in-plane switching, multi-domain vertical alignment, and patterned vertical alignment, have wider angles of view than TN panels can offer.
Contrast ratio: This term refers to the difference in light intensity between the brightest white and the darkest black that an LCD can produce. Look for a contrast ratio of 400:1 or better - with anything lower, colours may wash out when you turn up the brightness and may disappear when you turn it down. However, higher is only better up to a point. Contrast ratios over 600:1 are unlikely to provide any advantage, and monitor vendors are likely using "fuzzy maths" to calculate those values, anyway.
When shopping for an LCD, your best bets are to check independent reviews such as those in PC World's and Good Gear Guide's Reviews sections and to trust your own eyes.