Fill layers and Adjustment layers are special types of layers. Adjustment layers let you manipulate a layer's lighting, color, or exposure. If you're mainly interested in Elements to spruce up your photos, you'll probably use Adjustment layers more than any other kind. Adjustment layers are great, because they give you the ability to undo or change your edits later on if you want to.
You can also use Adjustment layers to take the changes you've made on one photo and reapply those changes to another photo (see the box below). And after you've created an Adjustment layer, you can limit future edits so they affect only the area of your photo covered by the Adjustment layer.
You'll find out much more about all the things you can do with Adjustment layers in the next few chapters. For now, you just need to learn how to create and manipulate them.
Fill layers are just what they sound like: layers filled with a color, a pattern, or a gradient (a rainbow-like range of colors). There's more about gradients on Section 12.3.
Digital photographers should check out the Photo Filter Adjustment layers. They let you digitally make the sort of adjustments that you used to do by attaching a colored piece of glass to the front of your camera's lens. You can read more about what you can do with photo filters on Section 8.2.