Nick Barker's Photoshop layers overview
In Photoshop layers allow you to work on one element of an image without disturbing the others. Think of layers as sheets of acetate stacked one on top of the other. Where there is no image on a layer, you can see through to the layers below. You can change the composition of an image by changing the order and attributes of layers.
About the background layer
When you create a new image with a white background or a coloured background, the bottom-most image in the Layers palette is Background. An image can have only one background. You cannot change the stacking order of a background, its blending mode, or its opacity. However, you can convert a background to a regular layer by double clicking on it.
To add a new layer
Click the New Layer button in the layers palette.
Linking layers
By linking two or more layers or layer sets, you can move their contents together.
To link layers: Select the layers you want to link in the layers palette. Click the join icon (looks like a chain) on the layers palette.
Setting layer opacity
A layer's opacity determines to what degree it obscures or reveals the layer beneath it. To set a layer's opacity select a layer and use the capacity slider on the Layers palette, the higher the percentage the greater the opacity.