Layers are a really useful feature of Photoshop, however, all too often I meet people who hear about them and then put off using Photoshop yet they are quite easy to use. Once I explain what they are for and how they work these same people then give it a go and get some great results.
Layers can be used to create composite photographs or images where layers can be added on top of each other to build up one image. I have also used layers to take someone looking the wrong way or blinking in one shot and adding to another. You can also add text, shapes and use tools like brush for drawing freehand on it. You can also add adjustment layers allowing you to change exposure, brightness, colour and other settings. The great thing about all these layers is that they do not permanently affect the original image or photograph. These layers can be turned on and off, if you don’t like them you can delete them.
Taking something like the adjustment layer you could have a layer that applied a sepia colour, another with black and white, you can then have both or turn them on and off to see what works best, great if you are trying to decide what works best or run past someone else. Why not have two different sepia layers with different levels of adjustment for comparison.
I have created a number of video tutorials but here is the first on understanding layers so you can see what they do.