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 and destroyed on demand. Where people can be created and destroyed with the click of a mouse. A world made for him. A world he made.
I think the title says it all.
(Funny thing is, I just disabled the AI, and I got this. Didn't even mean to. Was going to set up some NPCs. But I saw the Heroic pose, and I saw the lighting and such... just had to take screens. And now they're here for your enjoyment).