Contrast ratio: An HDTV's contrast-ratio rating is supposed to measure the difference between a TV's darkest blacks and brightest whites--the idea being that we find images with a high visual contrast more realistic. Unfortunately, since manufacturers self-report contrast ratios without any standardized testing guidelines, contrast ratios aren't a reliable indicator of a TV's picture quality in the slightest. Related:
Response time: Response time measures the time it takes for a pixel in an LCD to turn from one color to another--either from black to white to black again, or from one shade of gray to another shade of gray. This spec is useful for making sure you have a monitor without a "smearing" or "blurring" effect around moving objects.
However, unless you look at the fine print, it isn't always clear which measurement the manufacturer reports (the black-white-black transition takes about twice as long as the grey-grey transition), and these days, response times are generally good enough that most people don't need to worry about them. Related: