Fundamentally it’s a wiki like any other. But there is a cool layer on top of it that could be revolutionary (although like many Web 2.0 concepts will probably fall short and just be “cool” – we can hope). The interface allows you to create “situational applications” that can link different components together with the ease of a wiki.
It doesn’t really make much sense just reading about it so go watch the video about it.
On a related note, you can now get snapshots of PHP 6.