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.
So, what is an adjustment layer? Open your Photoshop, load any image, and then look at the bottom of the Layers panel. You see the following:
From the many buttons you will see there, I want you to click on the fourth one, looking like a black and white...