OS X Lion has three new features designed primarily to simplify workflow, and make it easier to work with documents and applications. The features range from the simple, making us wonder why it didn't exist in previous generations of OS X; to what is probably the most difficult, at least in terms of the underlying code, although it appears as a simple user interface element to the end user.
This is the easiest new feature, and after you read about it, you'll probably shake your head and wonder why it took Apple so long to implement it. Essentially, Resume remembers where you are when you turn your Mac off. When you start your Mac back up, everything will resume right where you left off. The applications you were using will be open, the documents you were working with will be there, and your windows will be just as you left them.
This will require you to unlearn a few common bits of Mac etiquette, such as closing applications before shutting down, or closing document windows. On the other hand, you'll be able to just hit the shutdown menu and walk away, knowing that everything will be just as you left it when you turn your Mac back on.
Some applications have had an automatic save function for some time. But if you're like most Mac users, you tend to save documents frequently (or at least occasionally) as you work on them, to prevent that gut-wrenching feeling you get when the system locks up and you lose hours of work because you forgot to save it.
Auto Save is a system-wide capability that will save any open document as you're working on it. You can forget about pressing command+S, which, by the way, is probably the key combination I use more than any other.
Auto Save doesn't create new copies of a document; instead, it saves the changes you've made. This keeps your disk from becoming cluttered with multiple versions of a document. But just as important, Auto Save lets you quickly revert to what the document looked like when you opened it. Auto Save also lets you lock a document, so that no changes may be made to it. The lock feature is an easy way to create document templates, or to make read-only versions.
Because Auto Save was already creating information about what changed in a document since the last save, it was a simple conceptual leap to add the ability to access and create document versions. While the idea is simple, having a version control system that anyone can use is actually a very large undertaking. Software developers have spent years trying to perfect revision control systems, so having one that us mere mortals can use is amazing in itself.
Versions uses the same type of interface as Time Machine. You could even think of versions as Time Machine for a single document. It uses the same "starry night" user interface as Time Machine, and displays the current document and past versions side-by-side. You can see what's different between the versions, and select the version you want.
When you decide to share a document with others, Versions will strip out all of the past versions and save just the current document. That allows you to share without others being able to see what you went through to create the document.
Auto Save and Versions
Together, these two new OS X Lion features should fundamentally change how you work with documents. Of course, you can continue to work using your existing habits; Lion will purr right along with you. But if you don't mind the OS watching over you and protecting your work, Auto Save and Versions will be a real benefit.