What is a layer?

Layers are like stacked transparent sheets of glass or slides on which you can create images..
Layers are stacked one above another and are seen as if are you are looking from the top down
through a stack of transparent slides
An opaque layer near the top of the stack will block everything below it from being seen.
You can work on each layer as if it were an independent document
The bottom layer (called Background) is special. It is locked and can't be manipulated until you change it's name.
A layer may duplicate the pixels from your whole photo or just a selection from part of a photo.
A layer may be all or part of another photo.
A layer may contain computer instructions on how to modify the layer(s) below it. No pixels, just instructions.

Which programs use layers?
Photoshop, Photoshop Elements, Paintshop Pro, and several other programs.
Programs that use layers are generally the more powerful photo editing programs.

What are layers for?
Layers let you add components to an image and work on them one-at-a-time without changing the original.

How may layers be manipulated / controlled? The controls are found in the layers palette.
Layers may be duplicated
New blank layers may be added to your layer stack
A blank (no pixels) layer may be filled with a single color, a gradient, or pixels from other photos.
Layers may be deleted
Layers may be kept, but turned on and off.
The opacity of a layer may be adjusted from invisible to full strength.

The blending modes of a layer may be adjusted. A blending mode controls how a layer interacts with the layer just below it. There are many blending modes and their exact effect is difficult to predict. Just try them out.
Masks may be added to a layer to control which parts of a layer show through and which parts are blocked.
Layers have layer styles which add special effect to the pixels on the layer.
Layers are retained across work sessions if: 1) You do not merge the layers in a stack. 2) The file is saved in photoshop (pdf) or (tiff) format.

What are some practical uses of layers?
Correcting an over exposed photo
Correcting an under exposed photo
Use adjustment layers to make non-destructive and adjustable changes to your photo to: 1) Adjust exposure 2) Adjust contrast 3) Adjust color casts 4) Adjust contrast 5) Adjust color saturation 6)
Add a mat or border to a photo
Add text to a photo
Create a collage by combining the pixels from several layers (photos) into a single image.
Creating two or more versions of an image within a single file; 1) Very handy when preparing an image for competition because you can try out different versions of an image all in the same file. Then just use the layers that produce the version you like best when you print it.