Garry's Mod is a sandbox game. You must have at least 1 source game installed before you can play it. You basically can spawn any props you want out of any source game (you must have it installed). Same goes with NPC etc. You are given many tools to build with, or just to have fun with. Kind of annoying to explain, you'll have to see for yourself.
There are also many modes, such as Sandbox (basicly building), Roleplay (most popular), Trouble in Terrorist Town (team vs team), Deathrun (kind of obstacle course), Prop Hunt (Hide and Seek) and so many more.
People who aren't creative won't like the game.