What is a Gaffer?

That last one might be an old-fashioned term.
Yes, believe it or not, sometimes the job is called "best person" instead.

To answer your question, the gaffer's job is to calculate the amperage needed for all the lighting specs, maintain the appropriate generator at each location, and wire the scenes so that no electrical apparatus is visible in the shots. The best boy (or person) is the primary assistant to the gaffer. A great amount of time on a movie set is needed to rig and adjust the lighting. Often the best boy/person (implementing the plans of the gaffer) will work on setting up the lighting for the next scene, while the gaffer attends to all the immediate needs of the scene being shot. The director of photography, who designs the lighting necessary to achieve the desired effects, works closely with the gaffer and the best boy.