The Gaffer is the chief electrical/lighting technician and works under the direction of the Director of Photography (D.P.).  The Gaffer is in charge of the placement and movement of the Lights before and after shooting.  He informs the Best Boy Electrician as to what needs to be done, and then the Electricians & Grips work in unison to “Light the Set” as needed for the shot. 

One explanation of the origin of  the term "Gaffer" comes from England and refers to street light workers who used a hooked pole or "gaff" to tend to the old Victorian gas lamps.

Another explanation as the origin of the word comes the 16th century and refers to head man in charge of workers.