If you have lots of layers in your Photoshop image, and need to compare various editing stages, then use Snapshots
Snapshots are created using the History palette, by clicking on the small camera button.
Once a Snapshot has been made, it gets added to the top of the History palette. Rename them by double-clicking on them to help remember what changes were made.
Be warned however, because Snapshots aren’t saved when your Photoshop image is closed down!