Garry's Mod is a sandbox mod for the Source Engine. Unlike normal games there aren't any predefined aims or goals. Players are given tools and are left to entertain themselves.
The player is in a world where objects can be created, destroyed and manipulated in thousands of different ways on demand. Anything can be achieved with the click of a mouse. Create robots, contraptions, traps, ships, cars, explosives, movies, catapults and even complex automated machinery with hundreds of extensive user-made modifications. This is all capable in Garry's Mod.