It's kinda like a sandbox game (using the Source engine from Valve) where you can manipulate the objects and physics of the game. You can use characters from TF2, L4D and Half-Life (you can also create your own models but that's more complicated). There are a lot of videos on youtube with this mod, some of them are pretty fun to watch.

Activision is the company that makes Call of Duty. Just want to remind everyone about that. 

Anyways, as Mr. Khan said, this applies to many more companies than just EA.

Oh neat. Were the spongebob voices just added after the the video was made or is it possible to add them into the game?

Also, response to the video, I'm an EA fan, they make good games, sure they pump out a bunch of shitty games, but they also release a lot of good games.. they do make a lot of games. Haters gonna hate :P

I think all the voices and sounds (expect the ones that are from the characters themselves, like the voices of the TF2 characters when they were on the bonfire) are added after the video is done.