Motion control is a technique used in motion picture special effects. The most common usage of it has been in miniature photography. A camera will be mounted on a computer controlled dolly, and move around a miniature (IE a spaceship), giving the miniature the appearance of movement. This technique can be seen in such films as Star Wars, Blade Runner, and 2001: A Space Odyssey, and was pioneered for the later film by Douglas Trumbull.
The chief advantage of motion control is repeatability. If one wants to make multiple passes on a shot (one for each color, for example), one simply resets the computer and has the dolly run through it again exactly.
Motion control was used to interesting effect in David Cronenberg's film, Dead Ringers. In it, Jeremy Irons plays twins, and Cronenberg did not want to be limited to generic, The Parent Trap-style split-screens. Instead, to great expense, raised sets were built to accomodate a motion control rig. The rig was programmed to execute a camera move exactly. For the first take, Irons played one twin. Then the camera was reset, and the shot was played out again, this time with Irons as the other twin. The end result? A long, perfectly smooth, steadicam-style shot with two Jeremy Ironses conversing.