Subversion
Subversion is a version control system usually used to store computer code. However it is good at storing any kind of textual document - like an author's work-in-progress.
Novello stores its data in subversion, which means that as the author, you can have the confidence to be as brutal as you like with your changes. You can always go back to what you had last week
Commit
A "commit" is when you send the changes you have on your machine to the Subversion machine. A new version will be created, which you will always be able to return to in the future. Do as many "commits" as you like. The more the better!