Adjustment layers are correction layers that adjust the color or tone of underlying layers, without modifying their pixels directly. Choose from these types of adjustment layers: Brightness/Contrast, Channel Mixer, Color Balance, Curves, Hue/Saturation/Lightness, Invert, Levels, Posterize, and Threshold.
All adjustment levels have equivalent commands on the Adjust menu. The advantage of an adjustment layer is that you do not modify the image layers themselves, so it’s easier to make quick changes to the color and tonal adjustments. Add adjustment layers to test various color corrections or to see how several corrections look when you combine them. Hide, delete, or edit adjustment layers.
An adjustment layer applies to all layers below it that are at the same level, as follows:
If the adjustment layer is at the main level (rather than in a layer group), it applies to all layers below it in the stacking order.
If an adjustment layer is in a layer group, it applies only to layers within the group that are lower in the stacking order.
To add an adjustment layer:
On the Layers palette, click the layer above which you want to create the adjustment layer.
To apply the adjustment layer to a selection, make a selection.
Choose Layers
> New Adjustment Layer, and then select the type of adjustment
layer.
A dialog with the name of the adjustment layer opens. The preview windows
show the image before and after changes. The Adjustment
tab shows the color and tonal correction settings to change.
Note: The General tab includes layer properties such as layer name, blend mode, and opacity. Change these properties here or on the Layers palette. For more information, see Editing Layer Properties.
To reset values to default settings,
in the Presets drop-down list choose Default. Or, double-click the adjustment layer
to open the Properties dialog, and click the Reset
to Default button 
.
Modify the settings on the Adjustment tab, and then click OK. The adjustment layer is added just above the previously selected layer.
Creating Raster Layers from Selections
Promoting a Background Layer to a Regular Raster Layer