Photoshop enables you to undo the last action you ve done and redo the last action you ve undone. Beyond that, Photoshop enables you to step backward through the last actions you ve taken, effectively undoing the changes you ve made. After stepping backward, you can step forward again, effectively redoing the changes. The History palette helps you track the steps you ve taken.
Windows [Ctrl]-[Z], Mac [ z ]-[Z], [F1]
Undo the last action, or redo the last undone action
Windows [Ctrl]-[Alt]-[Z], Mac [ z ]-[Option]-[Z]
Windows [Ctrl]-[Shift]-[Z], Mac [ z ]-[Shift]-[Z]
Windows [F12], Mac [F12]
Revert to the last saved version of the file
By reverting to the last saved version of the file, you can easily discard all changes you ve made to the file since you last saved it. You can achieve a similar effect by closing the file without saving changes, and then reopening it. The advantage of the Revert command, which you can issue by using this shortcut or choosing File Revert, is that Photoshop adds the reversion to the History palette, so you can undo it if necessary.