Adjustment layers let you experiment with color and make tonal adjustments without permanently modifying the pixels in an image. You can think of an adjustment layer as a veil coloring the underlying layers. By default, an adjustment layer affects all layers below it, although you can change this behavior. When you create an adjustment layer, the Layers panel displays a white box representing the adjustment for that layer.
Fill layers let you fill a layer with a solid color, gradient, or pattern. Unlike adjustment layers, fill layers do not affect the layers below them. To paint on a fill layer, you must first convert it (simplify it) into a regular layer.
Adjustment and fill layers have the same opacity and blending mode options as image layers, and you can move and reposition them just as you do image layers. By default, adjustment and fill layers are named for their type (for example, Solid Color fill layer and Invert adjustment layer).