Writer.app is a simple word processor.
It's designed to get your first draft banged out and on its way to the editing table. Which is a perfect complement to NaNoWriMo, if you're into that sort of thing.
So get writing. Well, download it first. Then get started. Make sure you have your coffee or tea made already, so you have no more excuses.
Writer.app (sometimes pronounced 'Writer dot app') is sort of like Khoi Vinh's software concept at blockwriter.com.