The goal is simple. You have thirty days beginning on November 1st at midnight and ending on November 30th at 11:59 PM to write 50,000 words. Say you have about 250 words to the page. Do the math and you'll find that it comes out to be, like, a 1000 pages. Or something. I may carried a number wrong, but math is not my strong point. In any case, assuming you fire off the necessary amount of words by the end of the month, you'll have yourself a beautiful novel.
Then again, it may not be that pretty. Part of the fun/torture of NaNoWriMo (there I go, doing it myself) is taking no time to edit and focusing on pure, nasty generation. According to the website, it's "a fun, seat-of-your-pants approach to novel writing," which translates into, "you'll probably hate what you wrote when you read it later." They even have the good hear to warn you that "you will be writing a lot of crap." But they insist "that's a good thing." And I'm inclined to agree with them.
As writers, we want to edit as we go, often causing problems for us as we compose. Our internal editor is shouting away at us in our minds, reminding us that "that's crap" we just smeared on the screen/page/napkin. But with NaNoWriMo (ugh) we accept it, nay, we embrace it!
So why do it? The official website offers a few reasons ("To stop being one of those people who say, 'I've always wanted to write a novel,'" or "To be able to make obscure references to passages from our novels at parties") and though they may not seem the most productive reasons, I assure you there is something to get out of it aside from a bunch of pages of a mess. By participating in NaNoWriMo, you're pushing yourself to become a more dedicated writer. You set goals for yourself (write 1,600 words a night, 12,500 a week, etc.). The most beneficial thing about National Novel Writing Month (haha!) is that you get to gag that internal editor and stuff him in the trunk until he learns to play nice. Or until you're done with your novel.
Now, off to write 1,600 words of garbage.