Well, according to the NaNoWriMo website (more or less paraphrased), the goal is to finish 50,000 words of a novel. Hopefully at that point, you finish the novel as well, but that's not a prerequisite for winning. I don't think I'll have a problem with hitting the 50,000 word requirement (and therefore winning), but my story's not going to wrap up at 50,000 words. Maybe 100,000, or even 150,000 but not 50,000.