Adjustment layers non-destructive way of working with the layers underneath. Once we have an adjustment layer, we can pretty much specify where the effect occurs by painting in that layer with the paint brush tool. These special types of layers are very simple to use and powerful in enhancing photographs.
Above image shows a photograph on which two Adjustment Layers are applied. Top two Layer names shows what exactly is happening to original Photograph. Levels and Brightness/Contrast Adjustments are applied on top of original photograph. In case we don't want any of these effects, just uncheck the layers.
Here are few options available as Adjustment Layers -
Levels actually correct tonal values in the image.
Brightness/Contrast lightens or darkens the image to get appropriate contrast in original photograph.
Hue/Saturation adjusts colors in the image to get different shades and intensity of colors.
Gradient Map maps pixels to the color in the selected gradient.
Photo Filter adjusts the color balance and color temperature of the image.
Invert produces a photo negative effect by creating a negative based on the brightness values of the image.
Threshold renders the image in monochrome with no gray, so that you can locate the lightest and darkest areas.
Posterize gives a flat, poster-like appearance to a photo by reducing the number of brightness values (levels) in the image, thus reducing the number of colors.
Overall beauty of Adjustment Layers is Power with Simplicity of Non-Destructive enhancements !!!