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. Garry's Mod provides tools to constrain objects together in various ways. Using these tools the player can create working contraptions.

The simplest example is a car. The player spawns an object to be used for the car's body then adds wheels by firing the tool gun at the object. Wheels are then controllable using the numpad on your keyboard.

You don't have to make moving contraptions though - want to build a fort out of wooden crates then set it on fire? Go for it! Want to build a house using fridges for the walls and mattresses for the ceiling then drive a jeep into it all? Go for it!