I still cannot tell what glitch is after landing on their site a few times. There’s lots of ideas laid out for how you could use it, but I can’t find a good use case for me.
I think it's cloud9 but easier and aimed at less-technical users. Like Processing. I gave it a brief shot because joelspolsky and that was my impression anyway.
It's a programming environment. You can use it for anything you can program a computer for. In other words: anything.