Photoshop documents are broken up into layers, which are stacked on top of each other. By default, there is only one layer, and it's called "Background".
There is a palette called "Layers" that shows what layers you have. The ones at the top of the list are the layers on top, and the ones at the bottom are the layers at the bottom. When you look at the picture, you are seeing the layer on top. If the top layer is transparent at all, then you will see the next layer down, and so on, all the way to the bottom layer.