Layers are like transparencies stacked one on top of one another. Each layer can have different images and effects. When the layers are stacked, the images appear as if they are all a single image.
Why use layers?
Layers allow users to work on separate images in order to modify sections without changing the entire image.
The example below contains three layers: the background yellow layer, the red square layer and the green circle layer. If, for example, the circle in this image were in the wrong place, you could easily change its location by selecting the circle layer and moving the circle without affecting the rest of the image.
What are the icons in the layers palette?
The Layers palette has various icons that control the functions of the layers in the image. Following is a description of some of the most useful ones:
Notice the red arrow on the Layers window to the left. It is pointing to an image that looks like an eye. This image acts as a show/hide feature for the corresponding layer. When the eye is open, that layer is visible. When the eye is shut, the layer is not visible.
The green arrow points to an image that looks like a paint brush. It shows the layer that is active, in other words, the layer which is currently editable.
The blue arrow points to an image that looks like a vertical chain link. This links two layers together. For example, since the square layer and the circle layer are linked together, moving the position of the circle in the image would also change the position of the square.
The purple arrow points to the new layer icon. A single-click on this icon will result in the automatic creation of a new layer.Credit to Dr. Mary Nicholson
