The elaborated history control is one of the most useful features in Photoshop, especially for beginners. Let's learn how to use it.
§ 1. Open an image of your choice and have some fun spoiling it as badly as you can.
§ 2. Display the History palette. (If you can't find it pull down "Window > History".) Every step you have made is listed there as a history state.
*By default, the palette can store up to 20 history states. The author recommends increasing this number.
§ 3. To get back for one or more steps, press [Alt+Ctrl+Z] one or more times. Note how it affects the History palette. To move forward in your history, press [Shift+Ctrl+Z].
§ 4. Keyboard shortcuts are very handy for navigating the recent history, while for substantial undos and redos we usually need the History palette. Click any state in the palette to move backward or forward in your history.
§ 5. To backup a key history state, you may create a history snapshot. Snapshots are shown at the top of the palette. The first one (in the movie, "focusing.jpg") is always created automatically when you open an image file.
§ 6. By default, our history is linear. What does it mean? Select an earlier history state and edit the image in some way; for instance, make a stroke with the Brush Tool. All the history states below the new step disappear!
§ 7. However, if you have made a "backup", snapshot is still there. Click it to return to the saved state. By creating a snapshot from a state, you are protecting it from the possibility of being deleted as you navigate and create new histories.