A Gobo (derived from Go Between or Goes Before Optics) is a thin piece of metal, wood, or glass used to used to modify the shape of the projected light. It is generally used with Ellipsoidal Reflector Spots, which normally provide a specific slot for gobo use. Old soda cans make a cheap replacement for custom made metal gobos, but don't last as long as professionally made gobos.
Plastic gobos—which are generally custom made—are available when a pattern is needed in color and glass does not suffice. However, these thin plastic films generally need to be used with special cooling elements to prevent melting them. A lapse in the cooling apparatus, even for just a few seconds, can cause an expensive gobo to be ruined.
Glass gobos can become very complex, and can make use of coloring, much like a stained glass window.
Most manufacturers produce a swatch book of their gobos, and like gels they are referred to by a number rather than name. For example, most manufacturers offer a gobo of a window, but they are all slightly different. So instead of calling it window, it would be identified as gobo 77143