Adjustment Layers

Here's a great intro video: link here.

But basically, think of an adjustment layer as a filter you lay over your image. But you can make this filter affect only certain parts of your image by painting on the mask. You can use adjustment layers to alter your images without making permanent change. That is, you can go back later and "turn off" the change if you want.

The main advantage of the adjustment layer is that it automatically comes with a mask. This means you can add the change (or "filter") just to a certain part of the image, instead of the whole image. You do this by painting with either black or white. See this curves demo to understand what I mean.

*rule of thumb with masking: White reveals the change and Black conceals the change.

CURVES:

You can use a Curves adjustment layer to brighten (dodge) or darken (burn) selective areas of an image.

For example, in the image below, I added one Dodge layer to brighten select areas. The result is an image that looks more dynamic and less flat:

In your adjustments palette, choose the Curves icon. Drag from the middle up for brightening, or down for darkening:

This automatically makes a mask for you. Invert the mask (command i) and then paint with a WHITE SOFT BRUSH on the mask, just on the areas where you want to see the change.

Remember, on a mask: White reveals, Black conceals. Above, I have painted with white JUST where I want the change to show through.

HUE SATURATION:
(to boost individual colors)
In the image below of the pomegranates, I used a hie saturation adjustment layer to isolate and boost just the reddish tones in the picture by moving the "saturation" slider to the right:
(Be sure to choose "reds" instead of "RGB" in the menu I've circled here -->)

This is what the adjustment layer looks like on your layers palette:

Because the adjustment layer's mask is white, the change is visible.
Remember, WHITE REVEALS, BLACK CONCEALS.

Similarly, you can reduce the red by moving the slider to the left.