Adjustment Layers
An Adjustment Layer works like a ghost layer. It is used to make non-destructive tonal changes to regular images. The clue here is in the term "non-destructive." Editing files, especially JPEG files, can be destructive, especially if it's done over-enthusiastically, repeatedly, or not very well. In such cases, the image (eventually) loses quality, unless it's a RAW file, which is always a copy of the original. I may add that, although this non-destructiveness claim certainly sounds impressive, it's actually hard to quantify with real-life examples, when compared with 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 Elements feature that can be used to limit the effect of any change, tonal or otherwise, to selected parts of the picture. It is an incredible way to lighten, darken, saturate, or desaturate...
