If you're a frequent user of the Steam PC application, then chances are you've come across Garry's Mod at one point or another. Garry's Mod (or Gmod for short) is a sandbox game built on Valve's Source game engine, which uses assets from their most popular games. By this I mean you have full access to characters, props, and maps from these games, and you may combine them in any way you see fit. Have you ever wanted to take characters from Team Fortress 2 and place them into the Half-Life 2 universe? Gmod has got you covered!
Garry's Mod has no real purpose other than letting you have fun messing with game objects; there's no true "game" here (although if you're crafty you could use it to code up your own online multiplayer game).