http://en.wikipedia.org/wiki/Grip_(job)
In the U.S. and Canada, grips are lighting and rigging technicians in the film and video industries. They constitute their own department on a film set and are directed by a key grip. Grips have two main functions. The first is to work closely with the camera department to provide camera support, especially if the camera is mounted to a dolly, crane, or in an unusual position, such as the top of a ladder. Some grips may specialize in operating camera dollies or camera cranes. The second main function of grips is to work closely with the electrical department to create lighting set-ups necessary for a shot under the direction of the Director of Photography.