I guess the easiest way to think of a layer is like a transparency for over head projectors. You know, the old style projectors people use to use for PowerPoint presentations?
Nowadays, of course, they have the fancy LCD projectors, but back in the old school days you had to lay these transparency sheets on a projector and it let you put drawings up on the screen. The neat part though was that because the light had to shine through, most of the transparency was exactly that, transparent.
That allowed for you to stack multiple transparencies on the projector and create a ‘composite’. A composite is a collection of multiple images or elements, arranged together to make a final ‘composite image’. Basically, it’s a stack of images arranged so that it made a new picture, one that didn’t exist before.
So let’s say you had a transparency of a tree. A transparency of a hill. A transparency of a sky. And a transparency of a cloud, and even one of a sun.
You could lay down the sky transparency first, then the hill. You would now have a scene with a sky and hill!
Next, you could add the clouds, the sun, and the tree. The final resulting image would be a beautiful picture. If you wanted, you could move the tree, and ONLY the tree to a different location. You could move the sun, or the clouds. You have a lot of power and flexibility.
Not only could you move an element, you could remove it altogether. You could take out the tree, or the cloud, for example.
Again, you could REORDER the elements and make an entirely new scene. For example, you could put the sun BEHIND the cloud by placing the transparency for the sun underneath the cloud.
You could do the same to make the cloud appear either behind or in front of the tree (which would have to be a tall tree).
You get the idea, you can do whatever you want. That’s the point. With each of these elements, you control the composite image and make it into whatever you want it to be.
The same thing applies to layers in Photoshop. You simply create layers, add elements to those layers (by draing on them, for example) and then you arrange the order of the ‘stack’ of layers. Layers on the bottom of the stack, are behind layers that are at the top of the stack.
That means that if a layer is at the very top, anything drawn on that layer will show up over top of anything else.
If a layer is at the bottom of the stack, it can easily be covered up by other elements that are on layers above it.
It’s so easy and so powerful. But you know what? It’s not just the layer order that controls how the final composition looks, you also have opacity and layer blending modes. But that, is for another article.