Gaff Tape (Sometimes referred to as spike tape when seen in 1/2" with and colors other than black) Is used to secure cables and other loose parts during the run of a show. It often looks similar to to duct tape, and like duct tape in can be ripped by hand, but unlike duct tape it uses a fabric as opposed to vinyl or plastic backing. It also differs from duct tape in that the adhesive is heat resistant, and leaves no residue when removed.
Gaffing down cables is the process of affixing them to the deck or another surface, usually with the intention of preventing actors or other people in the area from tripping over or otherwise disturbing the cables. Gaff, especially black gaff, is sometimes used to cover lights or shiny surfaces so as not to attract attention during a run.
Gaff is most commonly seen in 2" width, but can also be found in many other widths. Spike tape, the 1/2" colored variation of gaff is commonly used to denote the location of things on stage during rehearsals.