T 2 :
Guide to Adjustment LayersSo what are adjustment layers?
Im sure you've heard of the Adjustments menu under Image in Photoshop. Well adjustment layers let you perform those adjustments on an image but with much more flexibility.
Why would I want to put my adjustments on an extra layer?
Because that way you can always adjust the settings of the adjustment (e.g Hue) without damaging the image itself. This means you can easily change the settings at a later date or delete/hide the adjustment completely. Even after you've saved the image, closed it rebooted photoshop and lost all the history! You can also use a layer mask on an adjustment layer so the adjustment only affects certain parts of the image.
First steps in Adjustment Layers
OK. Open up an image you want to adjust. I've got a nice picture of a boat on a beach. Head down to the Layers Pallete and click the adjustment layer button (see below) and choose the "Hue/Saturation" option

You will get a dialog box which looks suspiciously like the Hue/Saturation dialog found in the Image -> Adjustments menu. Set whatever settings you like. I'm going to whack the Saturation way up to add some colour to this image. Once your done click OK.

You will notice a new layer above your image layer in the layers pallete, and, providing you made some adjustments in the Hue/Saturation dialog your image should look different. But wait! I've just decided I only want to make the changes on the sand! With the normal way of upping the saturation I would have to undo, make a selection, remember the numbers I used etc etc etc. But with Adjustment Layers its really very easy.
On the Adjustment Layer there is what looks like a layer mask, and it is! Except its an Adjustment Mask. Whatever is white on that then those areas get the adjustments applied to them. By default it is all white. Now I want the saturation on the sand, and only the sand, to be upped.
So grab the brush tool, set the colour to Black and choose a nice big round hard brush. Mines at 36px wide. Now select the Adjustment Layer Mask. To show you how this works make one huge swipe across the image. Now you can see that some of the image has the adjustment applied and some hasn't (see below).

This is all very well to illustrate how it works but its not what I want. So undo. Now paint in everything apart from the sand (ot whatever it is in your image you dont want adjusted). Below is the image after I've painted the sky and boat in so only the sand is over-saturated. I've also got a screenshot of the layers pallete so you can see what it looks like.

OK. Well that looks nice, but, I dont think its saturated enough. I am just dieing to over-saturate my sand. So, all I have to do is double click the Layer icon thing, this:

Then I can edit my settings, maybe add some Hue or Increase/Decrease the saturation. Click OK and my changes are made. All nice and simple and you dont actually affect the origanal image .
Here are a few quick examples of what you can do with various adjustment and fill layers (Which work pretty much the same):

Some nice over saturated, too yellow sand

Add a gradient map for some interesting results.

Use the Pattern Fill Layer to do this. Its in the same menu as the Hue/Saturation Layer Adjustment we talked about earlier.
thnx to : Adrian @ 13Dots for this tutorial