Oh my....where to begin.
Well, to be concise it's a massive online, interactive database. But it's so much more fun than that sounds like.
Basically for you, the individual, it's a place to:
A.) catalogue your stash: you can upload pics of your entire stash to flickr and then have those pics organized in your ravelry
"notebook." So you know exactly what yarns and how much of it is in your stash without rifling through it everytime. You can also catalogue your needles, etc.
B.) collect and store patterns: You can catalogue what books or magazine patterns you have, you can bookmark all your favorite online patterns straight to your notebook and organize it in a queue.
C.) have a virtual Stitch'n'Bitch: Ok here's where the fun is (for me). All those yarns you catalogued? Well Ravelry
automatically tells you everyone else who has those yarns in their stash, or what projects other Ravelers have made with that yarn to give you ideas and visual examples. You can also catalogue everything down to the dye lot if you want to search for someone
to help you acquire that last skein of whatever to finish your sweater.
And remember those patterns you queued up? Well Ravelry
also automatically links to everyone who has it in their queue, or who is working on it, or who has finished it. So you can browse and see how your pattern looks as an FO in different sizes and yarns before you start. And you can also PM those people to ask questions, etc.
I love it
There are also social groups to join (there's a KH one, an NPR one, a Hugh Laurie one, sports teams ones, sock knitters, etc etc etc) and that just basically connects you to other knitters with similar interests. Gets you more ideas. There are forums (fora?) as well in the same general vein as KH, but honestly I much prefer the KH forum. I sort of ignore the Ravelry
forum but I do love love love it as a resource and database and idea generator.
Hope that gives you an overview
(and get on the waiting list!
I got carried away, Jan beat me to it.