NaNo launched in 1999 and last year 325,142 participated. Since it's beginning over 250 NaNo novels have been published, including Rainbow Rowell's Fangirl and Marissa Meyer's Cinder.
Here's how it works. You go to their website, fill out a profile, hang out with other writers and commit to trying to write 50,000 words in a month. At the end of November, you stop. Not everybody makes the goal, so don't feel bad if you don't. There are badges that reward your best efforts. Display them with pride!
Fifty thousand words is a lot to write in a month. The great thing is, no one expects them to be brilliant. You just write the story straight from your head. It will probably be a mess. Like most first drafts, it might make you wince when you read it. But it might also be the beginning of a good book.
NaNoWriMo is a fast, fun way to prove you have a novel in you. It doesn't guarantee it will be a novel worth publishing. The value of the words you write depend on you learning to write them well. So if you start NaNo as a beginning writer, don't expect to create a blockbuster novel. Use it as a learning experience. Most authors start by writing books no one will ever read. Why not do that with a legion of new writer friends? And for experienced writers who are using NaNo to bring a third or fourth book into the world, who knows? It could be the next Eleanor and Park.