Yesssssss. Now those of us that wished for more Portal 2 faster can start to see some custom levels without resorting to playing through the co-op for the umpteenth time. Also, those of you that wished for harder Portal 2 levels (as it was several notches away from fiendishly hard) can start to see more of what you like, bless your masochistic hearts.
The Authoring Tools themselves look to be similar to the Authoring Tools released alongside (six months later than) Left 4 Dead, which in turn is similar to the Source SDK. Therefore, it has the full suite of various modding tools, such as the ever-so-elegant Hammer Editor, the Model Viewer and other such tools. The main bonus here is that you can actually make levels specifically for Portal 2, instead of resorting to the slightly-hackish methods of porting all of the content to the Alien Swarm SDK, which several enterprising modders figured out.
In related news, this looks to be the break the much-troubled Portal mod Portal: Awakening was waiting for. After several bouts of extreme developer drama, the new lead modder announced that they’ll be porting all of the Awakening content over to the Portal 2 engine, once the SDK is released. So, soon-ish, I assume.
The SDK itself can be accessed in the normal manner in which such materials are accessed, which is the Tools tab in your Steam library.