Adjustment Layers
On the surface, an adjustment layer behaves somewhat like a ghost layer. We use them to make nondestructive tonal changes to regular images. The clue here is in the term nondestructive. Regular editing on JPEG, TIFF, PSD, and PNG files can be destructive, especially if it's done over-enthusiastically, repeatedly, or just not very well. In such cases, the image begins to lose quality, unless it's a RAW file, which is always a copy of the original. I may add that, although this nondestructiveness claim certainly looks impressive on paper, it's actually hard to quantify with real-life examples, when compared with some so-called destructive forms of editing.
Even so, for many photographers, one of the huge advantages of using an Adjustment Layer is that it can also be used as a mask. A mask is a Photoshop feature that can be used to limit the effect of any change, tonal or otherwise, on specific parts of the picture. They are an incredible way to lighten...