GIMP Layers Tutorial
Preamble
If you've ended up here, I assume you are a novice with the GIMP and image manipulation programs generally. Everyone has to start somewhere, so why not here 
I found that when I was learning to use GIMP, with no previous image manipulation experience, the eureka moment was when I understood what layers were and how one could have an effect on another. Once I'd grasped that, the rest just fell into place through experimentation. I hope the following tutorial does the same for you.
The basics
Having opened the GIMP for the first time you were probably presented with a few boxes (dialogues), of which you recognised maybe one or two (if any!). Do not despair and don't give up, just follow these instructions. Close all the boxes except for the main GIMP toolbar and the Layers Dialogue.
The Layers Dialogue can be opened from the GIMP toolbar as follows File » Dialogues » Layers
Arrange the main GIMP toolbar and the Layers Dialogue so that they are to one side of the screen, which gives you most of the desktop for your image.
A four layer example
The example below on the left shows an image comprising four layers. The image on the right shows the Layers Dialogue with all the layers that make up the image.
The second set of images show the same layers, with layer 3 brought up to the foreground. This is achieved by selecting layer 3 (the green layer) in the Layers Dialogue and then clicking the Up button to bring the layer to the top of the stack.
Note: The layers have had some opacity added to them to make the example easier to understand, don't worry about what this is or how to do it yet, it's explained later.
Layers tutorial
Creating layers
The first thing I'll show you is an image that has two layers. If you want to create one to experiment with as this progresses follow these instructions. In the main GIMP toolbar go to File » New to create a new blank image.
Adjust the image to a reasonable size (200×200px) and click OK.
Duplicating the layer
You now have a 200×200px white image (white, assuming you opted for the default background colour). If you now look at the Layers Dialogue you will see this image is called Background.
Now go to the Duplicate Layer button in the Layers Dialogue and click for a duplicated layer, this will be Background copy.
Choosing a layer colour
With the Background copy layer still highlighted in the Layers Dialogue, go to the GIMP toolbar and double click on the Foreground Color button, which will open the Change Foreground Color tool.
In the HTML Notation box type in FF0000 (this is the hexidecimal code for red) and then click OK.
Adding colour to the layers
On the image toolbar go to Edit » Fill with FG Color, which will fill the Background copy layer with red.
Now in the Layers Dialogue click on the Background layer to select it.
Now repeat the above process with the Background layer, but fill the layer with blue. The hexidecimal code for blue is 0000FF.
Ok that's the work over, the rest of this tutorial will just involve tweaking the two layers you have hopefully just created.
Layer order
When you look at the Layers Dialogue you can see the red & blue layers. The red layer is at the top of the stack, so that is the one that shows up in the image.
It goes up, it goes down
With the blue layer selected in the Layers Dialogue click the Up arrow. You will now see that the blue layer is at the top of the stack and consequently the image is blue. Clicking the Down arrow in the Layers Dialogue will, as I'm sure you have gathered, move the blue layer back to the bottom of the stack.
Ok, all very interesting, but not very useful eh? Well you should now have a clear idea of what a layer actually is now. And you will need to know that to start using the GIMP.
Opacity
Ok, it's back to your art classes at school now. If you mix red and blue what colour do you get? And you all answered purple I'm sure
So with our red & blue layers it is very easy to do. Select the top layer in the Layers Dialogue, for this exercise it doesn't matter if the top layer is the red or the blue.
Now adjust the Opacity slider in the Layers Dialogue, until it is at 50%. As you will see the image has now turned purple. So now you have used one layer to have an effect on the overall image.
Opacity is not transparency! Opacity is in fact the polar opposite, it achieves the same thing though. Initially your layer will have 100% opacity. If you slide the Opacity down to 0% the layer would be 100% transparent. Glad we've got that sorted
Saving layers in GIMP
If you want to build up an image, each time you create a new layer, give it a name and save the file in GIMP's native format, which is .xcf. This will save all the layers and allow you to manipulate the layers at a later date.
Multiple layers
An image can be composed of a few or many layers depending on the complexity of the finished image.
Something more complex such as our logo requires more layers. The actual merged image is comprised of four layers, the text, the gold & the white backgrounds and the outer oval.
And even more layers
Of course to get the logo to the state above requires a good deal of image manipulation, which in turn involves a few more layers, which are then discarded once they have performed their task on the image.
For instance the bump mapped outer oval and golden background require Gaussian blurred layers to bump map them with. The inner oval that is made up from the golden oval and bump map was duplicated from a datum image. In total there were nine layers created for one image!
Bump mapping and gradients
There are some indepth tutorials on creating bump maps, gradients and other tasks with GIMP, which you can access from the menu at the top of the page.
Get to grips with layers first and then try them out. Experiment and enjoy.
