Sometimes the gaffer is credited as Chief Lighting Technician (CLT).
Experienced gaffers can coordinate the entire job of lighting, given knowledge of the time of day and conditions to be portrayed, managing resources as broad as electrical generators, lights, cable, and manpower. Gaffers are responsible for knowing the appropriate color of gel (plastic sheeting) to put on the lights or windows to achieve a variety of effects, such as transforming midday into a beautiful sunset. They can re-create the flicker of lights in a subway car, the motion of light inside a turning airplane, or the passage of night into day.
Usually, the gaffer works for and reports to the director of photography (the DP or DOP) or, in television, the Lighting Director (LD). The DP/LD is responsible for the overall lighting design, but he or she may give a little or a lot of latitude to the gaffer on these matters, depending on their working relationship. The gaffer works with the key grip, who is in charge of some of the equipment related to the lighting. The gaffer will usually have an assistant called a best boy and, depending on the size of the job, crew members who are called "electricians", although not all of them are trained as electricians in the usual sense of the term. Colloquially they are known as 'sparks'.
Many gaffers are expected to own a truck complete with most basic lighting equipment and then rent extra lighting equipment as needed.
Early studios were "available light" only, so there were articulated mirrored panels in the roof of the studio buildings that could be pushed from the floor by long "gaff" poles to bounce the sunlight to where it was needed on the set. Because the Earth moves continuously these hinged panels would need to be gaffed after each take. Once electric lighting instruments became the standard equipment, the light operators were known as electricians while the older, more experienced lighting technicians were still known as gaffers. Eventually it came to mean someone in charge of lighting.
Also posited: early films used mostly natural light, which stagehands controlled with large tent cloths using long poles called gaffs (a gaff is a type of boom on a sailing ship), or a pole with a hook on the end to assist in bringing nets or large fish aboard.
It should also be noted that gaffer tape, an adhesive tape used on the theatrical stage and the film set may have its name derived from the job of gaffer. It is known for having many uses, and its wide-spread utilitarian use could be easily likened to duct tape.