When you work with documents in macOS, each time you save you are saving a new version of that document. You can revert to a previous version of that document, or grab some items from a previous version that you may have deleted in your current version. This feature works in all Apple apps and many third-party ones as well.
Uninstalling apps is easy in Mac OS X Lion. If you have purchased an app in the Mac App Store, you can use Launchpad to quickly remove them. Otherwise, you can still use the basic techniques of simply dragging an application to the trash, or running its uninstaller program.
Preference files store your preferences and data for each application. Find out where they are and what they look like. In some cases you may want to delete or restore an old copy of a preference file.