Resume, one of the new features in OS X Lion, addresses a common complaint among users of desktop OS users. Traditionally, quitting an application in Mac OS X (or any other desktop OS) has meant starting over from square one when you next launch the app. This has also made restarting a Mac particularly painful for users who run multiple applications at once, because after a restart you'll generally have to relaunch them manually, then reload all documents you may have been working on before the restart, resulting in several lost minutes of productivity.
That's changed in OS X Lion. Apple has taken a page from iOS, where iPhone and iPad apps usually load up right where you left them, and brought the feature to the Mac. Quitting an application no longer means a laborious hunt for the document you were working on before you quit, because it loads up automatically. The old warning Safari used to give you about closing a window with multiple tabs is now moot, because the next time you launch Safari those tabs will reload automatically.
Resume is a great usability feature, and it really does make working on a Mac feel faster than ever before thanks to automating processes that used to require significant amounts of user intervention, just another way the Mac "gets out of your way" in Lion. It does have some troubling implications for security and privacy, however. If you had potentially sensitive documents loaded in Pages when you quit, they'll pop up again the next time you launch it. This applies to video applications like QuickTime Player, too, which will even launch movies stored on an external disc when you relaunch the app. If you watch, er, certain kinds of movies on your Mac, Resume might be a feature that could get you into a lot of trouble.
Thankfully, there are two ways of getting around those potential issues. First, you can disable the Resume feature entirely. In the "General" tab of System Preferences, there's a checkbox for "Restore windows when quitting and re-opening apps." Unchecking this completely disables Resume and reverts OS X to the traditional behavior where you have to manually relaunch documents and media files after relaunching an application.
Uncheck yourself before you wreck yourself
A less drastic workaround allows you to decide on a case-by-case basis which files you want to relaunch automatically after quitting an app, but as far as I can tell it hasn't been documented anywhere. Simply close the windows of any files you have open in the app before quitting it, and those files will not relaunch under the Resume feature the next time you launch the app. So if there's a document or movie that you'd rather not have come up automatically the next time you launch Pages or QuickTime, just remember to close the window before quitting the app. It's an extra step, sure, but if you want to use the Resume feature on your Mac without potentially giving Grandma a heart attack when your last session auto-loads in Safari, it's worth your while.
Thanks to Lion's autosaving Versions feature, even "Untitled" and unsaved documents will show up again. In the video below, you'll see that after restarting my MacBook Pro, all of the programs and files I had opened before the restart pop up again automatically, even a TextEdit document that I hadn't bothered saving.
The Resume process does take quite a while to get everything back up and running after restarting an older Mac, particularly if you're like me and saddled with a low-speed notebook-class hard drive. It's still faster than going through and re-opening everything manually, though, so if you need to restart your Mac during the middle of the day's work, go grab yourself a cup of coffee after the restart; by the time you head back to your desk, everything should be right where you left off.
Below is a video of the Resume process in action -- though I've edited out my doddering MacBook Pro's 3 or 4 minute boot time in the interests of brevity.
Apple OS X Yosemite