NaNoWriMo YWP is the website this wiki is about.
NaNoWriMo stands for National Novel Writing Month. NaNoWriMo is traditionally November. In NaNoWriMo, participants set a word count goal for themselves, then write furiously in their novels the whole month, trying to reach that word count goal. The emphasis is on quantity, not quality. Editing comes in December.
YWP stands for Young Writers Program. As the name suggests, the site only allows kids 17 and under (and 18-year-olds if they haven't yet finished high school.) The only adults allowed on the site are teachers and admins.