Working in an image-editing program without utilizing Layers is, from my perspective, like using a pro SLR and leaving it on manual all the time. You'll still be able to make quality images, but you certainly won't be taking advantage of all the technology that makes life easier!
While most photographers have adapted to the new technology in their cameras, such as autofocus, multiple-exposure modes and motordrives, many photographers still use image-editing software without enjoying the benefits of Layers. Like autofocus, though, once you start using Layers, you probably won't go back to your old ways.
What Is A Layer?
Think of a layer as a clear sheet of acetate over your photo. On this layer, you can add pixels in the form of image information—like paint on the acetate. Wherever there are no pixels (or "paint"), the layer remains transparent, and you can view the underlying layer or layers.
You can create multiple layers, remove layers, hide all or part of a layer, rearrange the order of layers, edit layers and blend the pixels of one layer into another in a variety of ways that give you different effects. These options all give you a lot of control and freedom when working on an image.
Besides pixel layers, you can have adjustment layers and vector or type layers, depending on the program you're using. Adjustment layers are empty layers that contain no pixels, yet they contain instructions that affect the appearance of underlying layers; they're like a filter on a camera's lens. They're used to make color adjustments to other layers without actually altering those layers.
Since adjustment layers contain information but not pixels, they can be edited, changing the adjustments as needed. You're able to make as many changes to the adjustments as you want because the effects of an adjustment layer don't become permanent until you flatten an image.
The advantage of using an adjustment layer is simple. Every time you adjust colors, contrast, brightness, saturation, etc., to the image itself, you're altering the pixels in your original image. The more changes, the more the pixels are altered, and the worse the quality of your image. Adjustment layers let you make those changes without altering pixels.
There are many ways you can use Layers to enhance your workflow. It can be something as simple as creating a new adjustment layer for color correction or adding a layer for cloning, or it can be something as complicated as adding a new element to fix an image defect.
To see how you might use a layer, see the image of Glacier Erratics taken in Yosemite National Park. I was using my view camera with a wide-angle lens and made a large movement with the front standard that ended up vignetting the top of the image. I always liked the picture, so I decided to rescue it from my error.
Because there was a large amount of cloning that needed to be done, I decided to work on a separate layer (make sure you then select the Sample All Layers option in the Toolbar for your Cloning tool); if I made a major error, I could just toss the layer and start again. Or, if my errors weren't so grievous, I could use the Eraser tool to remove the bad portion and then continue with the good. Otherwise, if I messed up on the if I messed up on the actual photo, I could be in serious trouble.
Using Adjustment Layers
Adjustment layers are used to adjust pixel layers in a nondestructive manner (that is, no pixels are actually changed until the photo is flattened). There are two ways to create adjustment layers in Photoshop. You can access them in the top menu via Image > Layer > New Adjustment Layer or by clicking on the Adjustment Layer icon on the bottom of the Layers palette (the circular symbol with black and white halves) and selecting the desired adjustment, including Levels, Curves, Brightness/Contrast, Hue/Saturation, Color Balance, Selective Color and Channel Mixer.
To readjust an adjustment layer, just double-click on the adjustment layer's icon and the dialog box will reopen with the current settings. Change those settings and click OK. If you decide you don't want those adjustments anymore, hold down the Alt/Option key and you'll see the Cancel button change to Reset; click it. You can even throw the layer into the trash with no damage to your pixels. Another advantage to having the adjustments take place on a separate layer is that you can quickly reduce the effect of the adjustment by lowering the opacity of that layer in the Layers palette.
Adjustment layers, by design, affect all layers below, so you can make the same adjustment to multiple layers at once. You can use adjustment layers to only affect one pixel layer in a layer stack by attaching a specific adjustment layer to just one layer. To do this, you need to have that layer active when you make your adjustment layer.
If you're making the adjustment layer from the icon on the Layers palette, hold the Alt/Option key as you choose the adjustment layer. This brings up a dialog box (the dialog box automatically appears if you make the adjustment layer from the Layers menu), where you check the box Use Previous Layer to Create Clipping Mask. This will attach the adjustment layer so it only affects the one layer. Notice when you do this that the icon is shifted a bit to the right, and there's an arrow indicating the layer is affecting only the layer below.
Layer Masks And Adjustment Layers
Adjustment layers affect the entire layer, making global or overall changes. If you want your adjustment to affect only a specific section of your image, you can use a mask that's created with every adjustment layer (by default, it's represented by the little white box to the right of the Adjustment Layer icon). In masks, keep this in mind, "White shows, and black hides." The default mask is all white, which means whatever adjustment you create, it will change the whole image equally.
But if you fill that mask with black by going to Edit > Fill > Use Black, the effect is hidden, blocked by the black mask. Select a Brush tool and white paint to paint in the change exactly where you want it to appear on the image (all these changes still take place only in the layer mask). If you want the effect to be strongest in one area, make that portion of the mask white. If you want the effect removed in another area, paint with black. And if the effect needs to be a little subtler in another area, paint with a shade of gray (or you can vary the opacity of the brush to make either white or black become gray).
Another benefit of working with adjustment layers is you can use Blend modes, which govern the way overlapping layers mix with each other. For photographers, the two most useful Blend modes are Multiply and Screen. These Blend modes will work in conjunction with unadjusted adjustment layers to change the exposure of your images (they only need a layer to work).
You can do this by first creating an adjustment layer without any adjustments made. Just click OK as soon as the dialog box opens. At this point, you have made no visible change to your image. Now change the Blend mode (the default is Normal on the Layers palette) by clicking on the little arrow to the right of Normal to get a drop-down menu. Select Multiply and the image will get one stop darker; choose Screen and the image will get one stop lighter!
If you want more than a one-stop change, duplicate the adjustment layer. If you want less than a one-stop change, lower the opacity of the adjustment layer. You'll find you have amazing control. And remember, you can paint on the layer mask of this adjustment layer; to change what's light or dark in the mask, you have the Photoshop equivalent of burning and dodging from the chemical darkroom.
These are just a few ways you can take advantage of layers. So, open up that Layers palette and experiment! Once you practice a bit, I'm convinced you won't go back to working on single-layer images again.