Gaffer tape, sometimes shortened to gaff tape (especially by theater and photographic professionals), or made possessive, as "gaffer's tape", is a strong, pressure-sensitive, cotton cloth adhesive tape. It is an essential, all-purpose tool on theater, film and television productions as well as live performances and any other kind of stage work.
The most common use for gaffer tape is securing cables to the stage floor or other surface, either for safety or to keep them out of view of the audience or camera. Camera assistants use short strips of different colors to lay blocking markers for actors. Similarly, a narrow version of gaffer tape, called spike tape, is used in theatre productions for floor layout. It is also used whenever a quick ad-hoc fix is required, from temporarily attaching fixtures or props, to salvaging a broken piece of production equipment. In the absence of console tape or artist tape, live sound engineers may use a strip of white gaffer tape along the bottom of a mixing board, to label the channels used for a particular show.
The tape is manufactured in many colors, including fluorescent and custom colors, but the most common variety is matte black or dull grey. A matte finish keeps the tape from reflecting light so the tape blends in with the typical stage floor of a theatre. It is easily torn by hand so no cutting tools are necessary. The adhesive used is a high quality synthetic rubber which leaves little or no residue when removed. It usually comes in 2"-wide rolls, and the cloth composition allows a consistent tear, which means it easily tears into two 1" strips, if needed. Gaffer tape should not be compared to duct tape, a far cheaper product which does not tear cleanly and leaves a residue when removed. 
It is likely named for the gaffer, the head of the lighting department on a film crew. When cables are taped down on a stage or other surface, either for safety or to keep them out of view of the audience or camera, they are said to be gaffed or gaffered.