Auto-Save and Versions are two system-wide features that Apple introduced in OS X Lion. Both features attempt to ensure that you don't lose any work you've performed on a document because of events beyond your control, from power failures to forgetting to save your work. (I don't know about you, but until my morning coffee kicks in, I consider just about everything to be beyond my control.)
Screen shot courtesy of Coyote Moon, Inc.
Auto-Save and Versions don't work with every application on the Mac, but as new apps are introduced and older apps are updated, both features should be supported by most document-based apps.
You can find out more about these new document management features in the guide: Using Auto-Save and Versions in OS X.