Photoshop is infamous for having one level of Undo. That sounds very limiting (and it is) until you realize the amazing flexibility and forgiveness of the History panel.
The History panel keeps track of every operation you do so you can wander back and forth through your recent Photoshop actions, just like going back in time—and forward again if necessary.
You also have access to the History Brush, which paints on an image using a former image state instead of color. It’s kind of freaky.
And, of course, the standard Undo, Redo, and Revert commands, though limited, are very useful in any project.