Getting To Know Layers
GIMP 1.0 introduces a concept called layers. Layers allow you to treat an image as a series of planes stacked on top of each other, and it gives you fine control over how each plane contributes to the final display and composition of an image.
Layers in an image can be any size. They can even exceed the dimensions of your image! However, your overall image size determines how much of any particular layer you might see at a time.
Layers also have special attributes associated with them. You have the ability to toggle the visibility of a layer, so that it does not contribute to the final image. You can change the Layer Mode of a layer, which affects how the layer interacts with the layer below it. You can also toggle a link between two or more layers, so that they may be moved as a group using the Move tool.
The layers dialog, with the Ops menu expanded
The primary tool for working with layers in GIMP is the Layers dialog. This gives you an overview of all the layers in an image, and provides the toggled areas for linking and visibility. Visibility of a layer is indicated by an eye icon to the left of the layer's thumbnail. A link is represented by a four-headed arrow to the left of that.
From the Layers dialog, you can also change a few other attributes. The opacity of your currently selected layer can be adjusted from 100% (fully opaque) to 0% (fully transparent). At 50%, you can partially see the layers below the current layer.
This is similar to the channel operation of blending.
You can also toggle a layer's Preserve Transparency option. This insures that if your layer has regions of transparency, any operation you perform on the layer will maintain that same level of transparency. When Preserve Transparency is enabled, you can't paint into transparent areas or erase opaque areas.
I'm not going to get into Layer Masks and the details of all the Layer Modes just yet.