You may have noticed already that the Undo command acts as a toggle for Undo and Redo, so you can only Undo the last action taken. And the Revert command takes you all the way back to the state of your image when it was last saved. So what if you want to undo more than one action, but you don't want to completely revert to the last saved state? That's where the History Palette comes in.
The History palette in Photoshop is the command center for keeping track of changes to your images, undoing editing steps, and correcting mistakes. 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.
Open any image and look at the history palette now. At the top of the palette is a small thumbnail icon and the file name of your image. Whenever you open an image, Photoshop creates an initial "snapshot" of the image and lists it here in the history palette. A quick way to revert your image is to click this initial snapshot.