Imagine the ability to apply an effect to a group of layers in Photoshop, and then have the power to edit that effect later, while preserving the original layers. Adjustment Layers allow just that.
To create an Adjustment Layer, click on the arrow at the top-right of the Layers palette, and select New Adjustment Layer...
Select the type of effect desired, and then hit OK.
The properties window for the effect choosen will appear, where that effect can be customized. After accepting the values for the Adjustment Layer, all of the layers underneath it will be affected.
Double click on the Adjustment Layer to edit the effect again.
To make an Adjustment Layer only affect the layer directly beneath it, hold down Alt, and move the cursor between the Adjustment Layer and the underlying layer. When the cursor changes into two overlapping circles, click.
This will use the underlying layer's transparency as a mask for the effect. In other words, it will only apply the adjustment to the underlying layer.
When drawing on an Adjustment Layer, the areas that are colored White will be affected the most by the adjustment. Areas which are colored Black will have no effect, while gray areas will only be partially effected.
As seen on the left, an Adjustment Layer is being used to Colorize the leaf image Green. By selecting the the Paintbrush tool and setting the foreground color to Black, drawing on the Adjustment layer restores the original image's colors. Drawing with White on the Adjustment Layer would re-Colorize the leaf image. Try it!