The
eye reports what it sees to the brain about 20-25 times a second. This is the basis of how
movies work: you flash a whole bunch of images in front of the eye at at least that rate (20-25
hertz), and as far as the eye is concerned, it looks like fluid motion.
Now, let's take one of these frames that last for about 1/25 of a second (about 40 milliseconds). What happens if an object moves a significant distance during this period? The the object will not appear to the eye as a sharp-edged object, but will be blurred because it reflected light to the viewer's eyes from a number of positions. This is called motion blur.
It's a phenomenon that can also be observed in photographs, especially with low shutter speeds. Sometimes this is deliberately used to create special effects, like those "running city" shots that have lines where all the car's headlights have been.
It's also made its way into computer graphics. Without motion blur, images look to sharp, and they can cause a queazy reaction among some people, since they look unrealistic. There is even beginning to be hardware support for motion blur on 3D cards for PC's.