About layers
Photoshop layers are like sheets of stacked acetate. You can see through transparent areas of a layer to the layers below. You move a layer to position the content on the layer, like sliding a sheet of acetate in a stack. You can also change the opacity of a layer to make content partially transparent.
You use layers to perform tasks such as compositing multiple images, adding text to an image, or adding vector graphic shapes. You can apply a layer style to add a special effect such as a drop shadow or a glow.
Work nondestructively
Sometimes layers don’t contain any apparent content. For example, an adjustment layer holds color or tonal adjustments that affect the layers below it. Rather than edit image pixels directly, you can edit an adjustment layer and leave the underlying pixels unchanged.
A special type of layer, called a Smart Object, contains one or more layers of content. You can transform (scale, skew, or reshape) a Smart Object without directly editing image pixels. Or, you can edit the Smart Object as a separate image even after placing it in a Photoshop image. Smart Objects can also contain smart filter effects, which allow you to apply filters nondestructively to images so that you can later tweak or remove the filter effect. See Nondestructive editing.
Organize layers
A new image has a single layer. The number of additional layers, layer effects, and layer sets you can add to an image is limited only by your computer’s memory.
You work with layers in the Layers palette. Layer groups help you organize and manage layers. You can use groups to arrange your layers in a logical order and to reduce clutter in the Layers palette. You can nest groups within other groups. You can also use groups to apply attributes and masks to multiple layers simultaneously.
Video layers
You can use video layers to add video to an image. After importing a video clip into an image as a video layer or a smart object, you can mask the layer, transform it, apply layer effects, paint on individual frames, or rasterize an individual frame and convert it to a standard layer. Use the Timeline palette to play the video within the image or to access individual frames. See Supported video and image sequence formats (Photoshop Extended).
For a video on using layers, see www.adobe.com/go/vid0001.