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The History Palette It's important to understand how to undo changes and correct mistakes. Two ways for undoing your changes are Ctrl-Z/Command-Z for Undo and the File > Revert command. 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.
Try it now. Open any image, make some obvious editing changes such as changing the image size or cropping. Create a snapshot. Now click the first snapshot in the list. The image reverts to its original state. Click the second snapshot and you'll be returned to the edited version. Combined with the History Brush, snapshots can be a very powerful tool. Below the snapshots there is a dividing line, and a list of all the recent changes you have made to the image. You can undo any of these recent changes just by clicking the last change you want to revert to, or by dragging the tiny arrow slider that appears next to each state. The slider is useful if you're not sure how far back you need to go because it allows you to preview the changes as you move it up or down. By default, Photoshop only lists the last 20 actions you have performed on an image and anything older is purged from the list to allow more memory for Photoshop. If you have a lot of memory, you can increase this number through the History Options command in the History palette menu. If you'd prefer to keep more memory available for Photoshop, it's a good idea to get in the habit of creating snapshots of your image at key points throughout the editing process, then you can still revert to an earlier state.
There are some important things you should know about the history palette and Snapshots:
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Let's 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 a 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. You can add new snapshots to the history palette
at any point by choosing new snapshot from the History palette menu
or clicking the new snapshot 
button.
Along
the bottom of the History palette are three buttons. The first button
creates a new document and works just like the duplicate command When
you duplicate the image, all history states are deleted for the duplicate.
The next button creates a new snapshot. This is quicker than using
the menu command but it does not give you dialog box to name the snapshot.
The snapshots will automatically be named in numeric sequence. The
third button deletes the current state or snapshot depending on what
is selected in the palette.