I picked this up with a discounted rate from Nanowrimo in 2013 - I can't praise it enough.
I haven't scratched the surface at what this thing can actually do - having now written three novels on it.
Basics - it's a word processor. It breaks the work down into scenes and you can compile them into chapters on the (default) left side of the screen in a tree system or you can use the digital cork board. Add titles and notes there as you please so you know what each scene is about at a glance.
The big one is for self publishing however, you can export directly to epub or mobi files and add your cover pages etc.
Downsides - grammar isn't as clever as MS Word and if you're using the Mac version not all of the options are available to you.
What I'm currently doing is writing the first two drafts on it, as it's far easier to move scenes and chapters about. So really good for structuring the plot. I'm then exporting the book to Word for a polish, picking up anything Scrivener may have missed (not much) before sending it off for evaluation.
Another cool widget is the word counter, you can set a daily/session target - and a project target. It gives you a metre toward the daily target as well as telling you how many sessions you need to hit the project target. This isn't for everyone, but I find it helps with the motivation.
It's also reasonably priced compared to other word processors and has a home license, I had to buy it twice as it doesn't cross over from Mac to PC (typical), but I've since been able to add Scriviner to my lap top with no extra charge.
Their website has some really good tutorials too.