Layers come in many flavors, all of which have their own special purpose:
Image layers. These layers are pixel-based (see Pixels and Resolution)—in fact, some folks calls ’em pixel layers—and you’ll work with them all the time. If you open a photo or add a new, empty layer and then paint on it (Chapter 12), you’ve got yourself an image layer.
Fill layers. When it comes to adding color to an image, these layers are your best friends. They let you fill a layer with a solid color, gradient, or pattern—handy when you want to create new backgrounds or fill a selection with color. You can also use them to change the color of art that you’ve placed in Photoshop (Changing Color). Just like shape layers (which are explained in a sec), you can double-click a fill layer’s thumbnail anytime to change its color. The next time you’re tempted to add an empty layer and fill it with color (Filling a Layer with Color), try using one of these layers instead.
Adjustment layers. These ever-so-useful layers let you apply changes to one or all the layers underneath them, though the changes actually happen on the adjustment layer. For example, if you want to change a color image to black and white, you can use a black & white adjustment layer (Black & White Adjustment Layers) so the color removal happens on its own layer, leaving the original unharmed. These layers don’t contain any pixels, just instructions that tell Photoshop what changes you want to make (which is why you can’t use any of ...