Not only can the history palette undo and redo changes to your images, it can save snapshots of your work in progress so you can go back to an earlier state or compare multiple revisions of an image.
The History Palette
Photoshop CS2 Basics Lesson 2: Working with Documents
The History palette in Photoshop is the command center for keeping track of changes to your images, undoing editing steps, and correcting mistakes.
The History Palette
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