Until now, while explaining all the different types of corrections in Photoshop, we've been using a simple direct manipulation of the final render where we just make changes and apply it, thus permanently changing the image. This is what we call a destructive working method, meaning that the changes are permanent. And more importantly, if you save and re-open the document, the changes can never be undone, so you only can remove them by undoing (by pressing Ctrl+Z).
At this point, we're going to dive into a more professional method by using adjustment layers, a non-destructive method. The benefit of this workflow is that the changes you make to your image can be edited later, always preserving the original image.