What are layers in Photoshop?
Many novice Photoshop users asks themselves, what are layers?
They think it's something very complex. If you're one of them, be reassured because it is very simple.
What are layers? Layers are nothing but a stack of transparencies, that's all. See one layer as one transparent (as were used with overhead projectors in the old days).
You can pick up a transparent, draw something on it, take another transparent, draw something on that one. Now put the second transparant on top of the first. Put the stack of transparencies on top of a white paper.
Now, when you view it from above, you see that your two drawings are combined in to one image!
Do you get it?
Why would you use layers?
Imagine, you pick up a sketchbook and a permanent marker, and you draw a house, a tree and a dog on the same page.
Now that you have a good look at the drawing, you prefer the house in the middle and the tree to the left of the house. In the case of a sketchbook and a permanent marker, you will have to recreate the drawing.
Imagine now that you draw the house on a page of your sketchbook, but you draw the tree on a transparent. And the dog on a second transparent.
You place the two transparencies on top of the drawing of the house, et voila! If you look at it from above, you see the complete drawing.
Do you want the tree on the other side of the house? Just move the transparent where the tree is on!
Just flip the 'dog transparent' 180 degrees.
So, concluding, by placing each object in an image on its own transparency (read 'its own layer'), you can adjust the objects independently. And that is why you work with layers.
So, layers in Photoshop are nothing but a stack of transparencies seen from above on the screen, that's all.
What can you do with layers?
With real transparencies the things you can do are limited, but on a computer with a program like Photoshop the possibilities are endless.
In addition to being able to change the order and position, you can also distort, enlarge or reduce the content of a layer, change color, apply an effect, duplicate, hide, group, etc. Most of these options will be covered in other posts.