Basically, it's a challenge to write a 50,000-word novel within the month of November.
I tried it last year (I got up to about 30,000 even though I didn't upload it all once I realized I wasn't gonna make it). It's fun. Even if your finished product by Nov. 30 isn't some great work of fiction, you might create characters or plots for later, more serious writing or learn about your writing style (since you'll start to feel the pressure in that last week, for sure).
I probably sound like some nerd in this post, but I just wanted to post up the link early so people could check it out. Plus, if there are at least a few OKPs participating, we can keep up with each other, you know?