Flat-screen LCD televisions are very popular these days. They give a sharp bright picture, and they are portable and inexpensive. However, the technology is not perfect, and LCD televisions can have contrast issues.
Unlike a plasma screen, a LCD, or Liquid Crystal Display, does not emit its own light. The picture is lit up, so that you can see it, by fluorescent lamps at the back of the screen. If too much light is visible from the backlighting, this can cause "muddy" colours and blacks that appear grey.
Contrast is the ratio between the black and white parts of an picture, so a ratio of 500:1 would mean that the whites in your TV picture were approximately 500 times brighter than the blacks. A poor level of contrast means that both whites and blacks seem grey and less sharp, and that fine detail cannot be perceived.
Unfortunately, there is no industry standard for the measurement of contrast ratio, which means models can only properly be compared with those by the same manufacturer. Many modern televisions boast impressive contrast ratios, but this can be misleading, as any improvement above 2000:1 cannot really be noticed with the human eye. Rather than waste money on a LCD television with a huge contrast ratio, try watching your TV in a completely dark room. Or buy a plasma set, as these have better contrast ratios.