Layers - Blending Modes
Photoshop has limitless possibilities when it comes to what a specific single layer can do. Perhaps one of the funnest settings to play around with are the Blending Modes. Blending modes change how a specific layer will react with another layer. Either the layer's opacity changes so that the layer underneath it bleeds through, or perhaps the image brightened. It matters solely on what outcome the image is planned to have.
Regardless, blending modes can be a bit confusing at first, but is worth the effort taken to learn. At Team Photoshop, the staff has put together a tutorial explaining Blending Modes and their Uses. Very useful to the new user.