What is Garry's Mod?

I was searching around YouTube for Team Fortress 2 gameplay footage when I found this video: http://ca.youtube.com/watch?v=bpOjH6ooi8g

Can someone explain to me what is Garry's Mod? I really want to be able to create totally crazy scenes like those.

Also, as a quick side question, what's the song playing in the background of the vid?

it's a game in which you can build and change the area your in.. Building from scratch or adding items and stuff it's pretty fun!

The game uses what Valve source games you have on your computer and lets you use all the items from those games...I just recently made my avatar using it

This link will tell you about Garry's Mod.

Garry's Mod