From the site:
NaNoWriMo is a novel-writing program for everyone who has thought fleetingly about writing a novel but has been scared away by the time and effort involved.
National Novel Writing Month is a fun, seat-of-your-pants approach to novel writing. Participants begin writing on November 1. The goal is to write a 50,000-word (approximately 175-page) novel by 11:59:59 PM on November 30.
As you spend November writing, you can draw comfort from the fact that, all around the world, other National Novel Writing Month participants are going through the same joys and sorrows of producing the Great Frantic Novel. Wrimos meet throughout the month to offer encouragement, commiseration, and — when the thing is done — the kind of raucous celebrations that tend to frighten animals and small children.
In 2011, there were 256,618 participants, and 36,843 of them crossed the 50K finish line by the midnight deadline.