layer's in Photoshop CS5
At the simple level layer is a virtual transparent platform that hold some content that content could be text could be photo or could be graphic a file that has more than one layer these virtual platforms each holding separate pieces of content are stacked on top of the other and important property of these layers they are transparent by nature.
Advantages of layer
Here is some advantages to making use of layers when you are working in Photoshop. They allow you to isolate the individual pieces of art work and that means you are able to move and to edit the content of each layer independently without disturbing the rest of the image.