Photoshop CS6 Tutorial
How to Use the History Palette in Photoshop
The History palette has multiple uses. Without the history palette, Photoshop would have only one undo.
It is also a source for painting with the History Brush. For more info, see Painting With the History Brush.
Open an image in Photoshop (File > Open...).
Open the History palette by clicking on the History icon or by choosing Window > History.
The top line of the palette contains a thumbnail of the image and is a snapshot of the starting image. The second line indicates the last step (Open).
Choose Image > Image Size and set the size to 50 percent for width and height. Press OK.
Choose Filter > Filter Gallery. In the option's panel open the Artistic section and select Palette Knife. Adjust the settings as desired, then click OK to apply your changes.
Take a look at the History palette. Each of the steps you just completed are listed in the palette. Click the icon for each history step to undo and redo your changes.
At any point you can take a snapshot of the current progress by choosing New Snapshot from the palette's pull-down menu.