Hi Welles,
Photoshop saves every brush stroke, every selection, every anything you do to your image as a state on the History palette (though the state only remains on the palette until you reach the maximum number of states or you close the document). There are three ways to move among states of your image.
? To revert your image back to a state, you can click on any state's tile in the History palette.
? You can move the active state marker to a state on the History palette.
? You can press Command-Z to step back to the last state (just as you've always been able to do). But you can also press Command-Option-Z to move backward one state at a time, and Command-Shift-Z to move forward one state at a time.
In general, when you move to an earlier state, Photoshop grays out every subsequent state on the History palette, indicating that if you do anything now these grayed-out states will be erased. This is like going back to a fork in the road and choosing the opposite path from what you took before. Photoshop offers another option: if you turn on the Allow Non-Linear History checkbox in the History Options dialog box, Photoshop doesn't gray out or remove subsequent states when you move back in time (though it still deletes old states when you hit the maximum number of states limit).
If you're doing straight-laced production work all day (the kind of work for which a single Undo is perfectly adequate), you may want to avoid the History feature's heavy RAM overhead by changing the History States value to 1 in the Preferences dialog box (press Command-K). Similarly, you can turn off "Automatically Create First Snapshot" in the History Options dialog box (which you can find on the History palette's popout menu). You might also want to turn off these functions if you're going to batch-process a number of images using actions or the Automate "wizards" (because in these cases, History isn't necessary).
Now if you just want to make notes or annotations to yourself you can use the much overlooked Notes Tool, or even the Audio Annotation Tool (as shown in the screenshot below).