Judder is a television screen artifact that occurs when content recorded on film is shown on a television with a 60Hz refresh rate.
The standard frame rate for film is 24 frames per second (FPS). When you are watching a film on a 60Hz television, software in the TV or DVD player detects the incoming signal and fills in the missing 36 frames by repeating frames that your eye has already seen. The problem is that 24 doesn't divide evenly into 60.
If each frame is repeated twice, there will still be 12 missing frames. That's where 3:2 pulldown comes in. To ensure that there will consistently be 60 frames per second, the first frame is displayed on the TV screen 3 times and the second frame is displayed 2 times. The following frame is repeated 3 times, the next one 2 times, etc. throughout the film.
Because alternating frames are not repeated in a consistent manner, the picture on the television screen is actually a little jittery. In TV-vendor lingo, this is called judder. Most of us don't notice judder because a second goes by very quickly and we are used to viewing films on television with a 3:2 pulldown..
See also: soap opera effect, Fast Guide to Flat Screen TVs, 3-D TV, LCD TV