The tools are free for owners of the game on PC, and can be found in the Tools tab of Steam accounts. As with previous Source Engine mod kits released by Valve, you'll be able to employ all of the game's assets: single player and co-op levels, character skins, sound effects, music, and 3D models are all at your disposal.
Beyond the basics, Valve is including an updated version of Faceposer (facial animation software), some "examples maps and instances to help build new maps," and an "updated suite of command-line compiling utilities." That last part sounds dangerous, so we'll leave it up to you to figure out what it does.
