The Answer:

The gaffer is the head electrician in a movie crew; it's her job to manage the lighting, making sure the levels are appropriate for the desired effect in the scene. The gaffer is assisted by the best boy, and may have a crew of electricians.

A grip is also concerned with lighting, but from the mechanical side. Where the electricians are concerned with brightness, the grips handle shading, putting up filters, nets, flags, or other objects in front of lights, or setting up large frames to block the sun. Grips are also responsible for the setup and general stability of scaffolds, ladders, and other heavy objects that might come crashing down. The head grip is called the key grip.

—The Fact Monster