Layers are one of the most useful features available in advanced image manipulation programs (the GIMP, Fireworks, Photoshop, Illustrator, and a bunch of others use layers for just that reason). Layers make your image infinitely manipulable. They do so by separating elements of your image into separate sheets, each of which can be altered and stacked on top of one another. This way, when you select part of the image to move, you aren't moving the entire image, just the part that's on that layer.
To use layers with Fireworks, click Windows from the top tool
bar and select layers. The layers window represents each layer as
though it were a separate sheet of celluloid, each piled on top of the
next.
To see one layer alone, click the eye to make it disappear - when the eye isn't showing for that layer, the layer itself is hidden.