1 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.
2 A new layer always appears immediately above the active layer. If no layers are active, a new layer appears at the top of the layer list.
3 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 commands—Bring To Front, Bring Forward, Send Backward, and Send To Back. However, you can’t change the layer position of a background layer unless you convert it to a regular layer (unlock it or double-click to rename it).
4 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].
5 To minimize file size, you can flatten the image, which merges all the layers onto a single background. It’s a good idea to keep a duplicate with layers intact before you flatten the document, in case you have to make changes to a layer later.