It’s called Non-Destructive.  Unlike other programs that tell you to make a copy before you start editing, Picasa handles your edits without destroying your original.  As long as you use Picasa for all picture-related tasks, you don’t even need to save.  If you do need to see your edited picture in some other program outside of Picasa, they you do need to save (or export) your changes.  This video explains.