As you may know, the really cool thing about Lightroom is that its non-destructive editing. So anything you do is reversible and since all the adjustments you make are merely changes in the instructions to the catalog database for that image, your actual raw file or jpeg is never touched.
The History panel in Lightroom works differently from Photoshop, which stores history as a temporary file and when you close a file and reopen it, the history is gone. But in Lightroom, in the history panel which is locatd in the Develop Module, you will always see every adjustment you ever made since the day you imported the image, no matter how often you open or close the software. You can go back to any point in time by finding the state that you want to return to, and click on it.
I can go back to my last edit by clicking the latest entry at the top of the History Panel. But, if you go back to a previous state in the history and then make new adjustments, all the adjustments above that will disappear because they won’t be revelant anymore. If you do this by mistake, then hit ctrl/cmd Z to go back.
You can also use Snapshots to go back to a certain point in time and if you have a long list of history steps this is a good way to get back to a certain state quickly. I have another tutorial on that here.
Change the “Before” View
Here’s another helpful thing you can do. Sometimes when you are comparing before and after, you don’t want to compare you image in its current state with the way it looked when you imported it. So here’s a cool trick. Get into your before and after screen by clicking down here and then go to your history panel and find the state you wanted to compare to, and drag that onto the Before screen. Be careful not to actually click on it in the History or whatever you clicked on will become your current state. The other way you can do it is to find the state you want to compare with and right click it and choose Copy History Step Settings to Before.
Clear All History
Also, if you want to clear the history from an image, in the Develop Module go to the top menu and click on Develop > clear history. Or on the History Panel: click on Clear All at the top right of the panel. If you do this by accident, just hit Ctrl/Cmd Z to undo. If you want to clear the history for a group of images, select them in the film strip at the bottom — and if it’s hidden Hit F6 to reveal it — and then select Develop > Clear history and that’s it. Since the history is stored as a data file it doesn’t take up a lot of space on your computer and won’t slow it down, so there is no real reason you need to clear the history but you can do it.