Resume is one of the main new feature in Mac OS X 10.7 Lion. It is a system feature that borrows from iOS, where apps states are saved behind the scene, allowing you to immediately get back to the exact state of the app even when system reboots. This is the description for Resume on Apple’s Mac OS X Lion homepage:
Now apps you close will reopen right where you left off, so you never have to start from scratch again. And when you install software updates, you no longer need to save your work, close your apps, and spend valuable time setting everything up again. With Resume, you can restart your Mac and return to what you were doing — with all your apps in the places where you left them.
In essence there is no need to worry about having to save your files, and there is no need to close any apps before you put your Mac to sleep. Applications requires rewrite in order to take advantage of this feature. We expect most apps to take advantage of this new feature eventually.
We encourage you to adapt to this ‘new way’ of using your Mac. For some people however it is hard to break the habit of closing down apps and saving works before logging out. If you find this default Lion behavior annoying, you can disable it if neccessary.
To disbale Mac OS X Lion Resume feature system wide, open the System Preferences->General. At the bottom under “Number of recent items”, uncheck the box “Restore windows when quiting and re-opening apps”.
Resume is one of the useful feature of Mac OS X Lion and we discourage you from disabling it system wide. If you would like just one app such as Safari or Quicktime to start with a clean state without opening a bunch of old windows, use the shorcut key Option+Command+Q to quit the app. The next time you start the app, the last opened windows and tabs will not be resumed and you will start with a new app.