Layers let you move and edit different parts of an image as discrete objects. You can also hide individual layers as you work on other layers.
A new layer always appears immediately above the active layer.
You can make artwork on one layer appear in front of artwork on another layer by dragging layers up or down the stacking order in the Layers panel, or by using the Layer > Arrange subcommands—Bring To Front, Bring Forward, Send To Back, and Send Backward. However, you can’t change the layer position of a background layer.
To apply a layer style, select the layer, and then click the Add A Layer Style button in the Layers panel, or choose Layer > Layer Style > [style].
To minimize file size, you can flatten the image, which merges all the layers onto a single background. It’s a good idea to duplicate image files with layers intact before you flatten them, in case you have to make changes to a layer later.