From Literature & Latte:
Scrivener is a content-generation tool for writers that allows you to concentrate on composing and structuring long and difficult documents. While it gives you complete control of the formatting, its focus is on helping you get to the end of that awkward first draft. This is a word processor and project management tool that stays with you from that first, unformed idea all the way through to the final draft. Outline and structure your ideas, take notes, view research alongside your writing and compose the constituent pieces of your text in isolation or in context. Scrivener won't tell you how to write--it just makes all the tools you have scattered around your desk available in one application.