Every other tutorial you read, you read about layers. You might already know what a layer is. In the GIMP, layers are like individual images within one, stacked on top of one another.

The mode that you choose for your layer affects how it "interacts" with the layer underneath. If you change the layer mode for the Background layer, for example, nothing will be affected. If you change the layer mode for the layer on top of "Background" however, the look will usually change.

By default, each new layer is put "on top" of the layer before it. This, however, can be changed. As with normal image operations, right-click, but this time click in the Layers dialog on the layer that you want to adjust. You'll see a menu pop up with things like Raise Layer, Lower Layer, Delete Layer… you get the idea.

There are many reasons you might want to use layers. One is that you can do as many things on a layer as you want, and if the results turn out crappy, you can delete the layer! Yes, undo is a great feature, but there are only so many levels of undo you can backtrack with (without driving your system's load average way up). Another reason is that you could be working on an image with a white background

The "control panel" for layers is the Layers dialog. This is also called the Layers and Channels dialog, but one section is specifically for layers. Here's what it looks like when the Layers and Channels dialog is set to let you manage your image's layers:

You can think of transparent layers as just that: something you can see right through. You can then add stuff on top of that transparent layer that isn't transparent. Think of the lowest layer as the kitchen counter, and a transparent layer as a piece of Saran Wrap; when you add stuff on top of the Saran Wrap, that part is covered by whatever you put in it, so you can't see through that portion of the plastic onto that section of the kitchen counter. You can see the parts that aren't covered by whatever you put on the Saran Wrap, however, so parts of that Saran Wrap "layer" still remain transparent.

When you clear a layer (Ctrl-K or right-click, Edit, Clear), it becomes transparent and with the stuff I've done, I couldn't use that layer anymore. What I usually had to do was either undo the Clear operation, or delete that layer and start over with a new one. However, you can't clear the Background layer just like that; if you try, whatever is set as your background color will fill the image. If you try to "clear" the image using Ctrl-X, you're deleting the background layer so that makes your image empty. At first it appeared to me that I had made the background layer transparent, but a check in the Layers dialog told me that there were no layers. I couldn't create any new layers after that and so that image was rendered useless. Of course, the mighty Undo (right-click, Edit, Undo; Ctrl-Z) feature of the GIMP is an option, taking you back to before you deleted the background layer.

Using channels won't prove useful most of the time, but if you've cropped an image and then started to work on it and the brush that you use appears to be the wrong color or doesn't seem to do anything at all, try making sure that everything (Red, Green, and Blue) in the Channels dialog is highlighted.

Right now I don't know exactly what channels are, but if you take a look at the Channels dialog with an image in RGB mode open, you can see what the image looks like without the Red channel by making the eye on the line marked "Red" disappear. Click on it. Click on it again. Can you see what happens to the image? It should have taken all the red out, and if your image was a plain white one, it should have turned green-blue.

The central control panel for channels is in the Layers and Channels dialog as well, only this time you click on the "Channels" tab after the layers dialog box pops up. Here's what it looks like:

Indexed images have only one channel; you can change that by making the image RGB (which is the mode you should be creating and editing images in anyway). With X pixmaps, sometimes a channel might be unhighlighted and therefore disabled. That brings up another point. For the best results, make sure all three channels (Red, Green, and Blue) are highlighted.

Again, whenever you're doing something and it doesn't do what it's supposed to, check to see if all the channels (Red, Green, and Blue) are highlighted. I remember that once I was working on an image and used Gaussian blur on the Background layer and it stayed the same; I ended up doing about three Gaussian Blurs with nothing happening until I checked the channels dialog while the highlighted, active layer was Background. I can't remember whether it was only one or all of the channels that weren't highlighted, but once all the channels were highlighted again, the Gaussian blur worked perfectly.