Quite often when I mention Photoshop people say that they got stuck the moment layers were mentioned, and many do not even try it. It seems to imply some incredibly difficult thing to do. The good news it is not and have easily explained it so they can then go on to use them.
Layers are used to add photographs to another, add text, shapes, freehand drawing or painting and to make adjustments to exposure, colour, saturation and a lot more. The advantage of the layer is if you add something you can change it without affecting the original photograph. You can also try out adjustments such as brightness and contrast and easily revert back to the original, you can have variations on different layers so you can see how they look and run them by someone to see which one they prefer.
Here is a video of the explanation I use. Once you have seen it you will want to give it a go and there are some links below this video to take you to these tutorials.
Other tutorials about Photoshop layers include:
Add a photograph to an existing photograph.