If you are working with Photoshop it is very important to know what are layers and how to use them. I found on Adobe page very good article where are layers described.
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.
Transparent areas on a layer let you see layers below.
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.
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 panel. 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 panel. You can nest groups within other groups. You can also use groups to apply attributes and masks to multiple layers simultaneously.
Layers for non-destructive editing
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 non-destructively to images so that you can later tweak or remove the filter effect.
Layers panel overview
The Layers panel lists all layers, layer groups, and layer effects in an image. You can use the Layers panel to show and hide layers, create new layers, and work with groups of layers. You can access additional commands and options in the Layers panel menu.
Photoshop Layers panel
A. Layers panel menu
B. Layer Group
D. Expand/Collapse Layer effects
E. Layer effect
F. Layer thumbnail
Display the Layers panel
Choose Window > Layers.
Choose a command from the Layers panel menu
Click the triangle in the upper right corner of the panel.
Change the size of layer thumbnails
Choose Panel Options from the Layers panel menu, and select a thumbnail size.
Change thumbnail contents
Choose panel Options from the Layers panel menu, and select Entire Document to display the contents of the entire document. Select Layer Bounds to restrict the thumbnail to the object’s pixels on the layer.
Expand and collapse groups
Click the triangle to the left of a group folder.
Filter layers (CS6)
At the top of the Layers panel, the filtering options help you find key layers in complex documents quickly. You can display a subset of layers based on name, kind, effect, mode, attribute, or color label.
Filter layers options in the Layers panel
- Choose a filter type from the popup menu.
- Select or enter the filter criteria.
- Click the toggle switch to switch layer filtering on or off.
Convert background and layers
When you create a new image with a white background or a colored background, the bottommost image in the Layers panel is called Background. An image can have only one background layer. You cannot change the stacking order of a background layer, its blending mode, or its opacity. However, you can convert a background into a regular layer, and then change any of these attributes.
When you create a new image with transparent content, the image does not have a background layer. The bottommost layer is not constrained like the background layer; you can move it anywhere in the Layers panel and change its opacity and blending mode.
Convert a background into a layer
- Double-click Background in the Layers panel, or choose Layer > New > Layer From Background.
- Set layer options.
- Click OK.
Convert a layer into a background
- Select a layer in the Layers panel.
- Choose Layer > New > Background From Layer.
Any transparent pixels in the layer are converted to the background color, and the layer drops to the bottom of the layer stack.
Note: You cannot create a background by giving a regular layer the name, Background—you must use the Background From Layer command.
You can duplicate layers within an image or into another or a new image.
Duplicate a layer or group within an image
- Select a layer or group in the Layers panel.
- Do one of the following:
- Drag the layer or group to the Create a New Layer button .
- Choose Duplicate Layer or Duplicate Group from the Layers menu or the Layers panel menu. Enter a name for the layer or group, and click OK.
Duplicate a layer or group in another image
- Open the source and destination images.
- From the Layers panel of the source image, select one or more layers or a layer group.
- Do one of the following:
- Drag the layer or group from the Layers panel to the destination image.
- Select the Move tool , and drag from the source image to the destination image. The duplicate layer or group appears above the active layer in the Layers panel of the destination image. Shift-drag to move the image content to the same location it occupied in the source image (if the source and destination images have the same pixel dimensions) or to the center of the document window (if the source and destination images have different pixel dimensions).
- Choose Duplicate Layer or Duplicate Group from the Layers menu or the Layers panel menu. Choose the destination document from the Document pop‑up menu, and click OK.
- Choose Select > All to select all the pixels on the layer, and choose Edit > Copy. Then choose Edit > Paste in the destination image. (This method copies only pixels, excluding layer properties such as blending mode.)
Create a new document from a layer or group
Select a layer or group from the Layers panel.
- Choose Duplicate Layer or Duplicate Group from the Layers menu or the Layers panel menu.
- Choose New from the Document pop‑up menu, and click OK.
Sample from all visible layers
The default behavior of the Mixer Brush, Magic Wand, Smudge, Blur, Sharpen, Paint Bucket, Clone Stamp, and Healing Brush tools is to sample color only from pixels on the active layer. This means you can smudge or sample in a single layer.
To smudge or sample pixels from all visible layers with these tools, select Sample All Layers from the options bar.
Change transparency preferences
- In Windows, choose Edit > Preferences > Transparency & Gamut; in Mac OS, choose Photoshop > Preferences > Transparency & Gamut.
- Choose a size and color for the transparency checkerboard, or choose None for Grid Size to hide the transparency checkerboard.
- Click OK.