If you've been walking through the local electronics store, or browsing HDTVs online (and I'm sure you have), you may have noticed a lot of attention being payed to "120 Hertz," that is, the refresh rate of some newer model televisions. The refresh rate refers to the frequency with which the image on the screen is updated, or re-drawn. "120 Hertz" means that the image you are looking at on the HDTV is updated 120 times a second.
In the past, most TVs had refresh rates of either 30 hertz (30 or 60 frames per second). While this was fine for viewing broadcast or cable television shows that are filmed at 30 frames per second, it created some minor issues when watching motion pictures, which are filmed at 24 frames per second. In order to make the 24 frames of film match with the 30 frames of the TV, a process known as "3:2 pulldown" was employed, in which the extra frames were filled with repeated images. In scenes with high motion, the imbalance of the repeated frames could cause the movie to look jerky and not smooth. Whether it's truly noticeable is up for debate: true videophiles claim to be able to detect the problem, while the average TV viewer might never notice that anything funky is happening to their display.
For those who are bothered by the "3:2 pulldown," 120 Hertz is a welcome solution. 120 is a multiple of both 24 and30, meaning that the framerate of a film and the framerate of a TV show can be repeated evenly, and both types of video content will look its absolute best.
Is this really important? Is it worth spending a few extra bucks to get get? That depends on what kind of consumer you are. If you read the above information and thought "I've never noticed anything weird about watching movies on my TV," then maybe you wouldn't really care if you had a 120 Hz HDTV. In any case, now that you know what 120 Hertz means, you can shop more effectively and with a greater sense of what it is you're looking at, and what the stores are trying to sell you.