GaffTape, also called Gaffer's Tape, is a special kind of DuctTape made for theatrical use. Like DuctTape, it is extremely useful and often used for rather ghetto repairs and other things it probably shouldn't be used for. GaffTape differs, however, from DuctTape in two important ways:
Because it doesn't leave a nasty residue, it is "safe" for uses like gaffing things to walls, gaffing things to yourself, binding people, etc. In general if you want to tape something to something else that shouldn't be damaged, GaffTape is a better choice that DuctTape. This, in my opinion, makes GaffTape more versatile than DuctTape.
Also because of its non-relective backing, it actually has a "dark side".