LayersLayers allow the image editor to create the illusion of depth in an image by arranging the workspace into separate depth planes. On each plane or 'layer' you can add visual elements such as images, text or shapes and apply edits, visual effects or transformations that affect the content of that particular layer while leaving the other layers untouched. This allows for a much more powerful image editing process as you are in effect editing multiple superimposed visual elements then combining them to create a final image.
The layers in an image are organized in a layer stack. Each layer is assigned an index that determines the depth of the layer content in the image. A layer with a higher index in the stack takes visual precedence over the layers below. The content in a higher layer will appear before or block the content in a lower layer. To get a practical visualization of a layer stack you can consider multiple stacked transparent plastic sheets with drawings on them. If we assign an index of zero to the bottom sheet then the topmost sheet, closest to your eye, would have the higest index. The content or visual elements in the a sheet would hide everything in the lower sheets or layers. Areas in a layer that are transparent reveal the content of the layer below it. Transparency or opacity are important properties when working with layers. The Eraser Tool, selection tools and Layer Masks are used used to selectively erase or reveal parts of a layer to create depth and interesting image compositions.
You should never think of a layer as being the content or object it contains. A layer that contains a red square is not a red square, it contains a red square. A basic layer is an abstract object that performs the primary function of indicating to the image editor the depth it should render the visual content it contains. Other specialized layers such as adjustment layers simply perform functions on other layers, like changing their color information. So while Adobe Photoshop classifies layers that contain editable text as 'Type Layers' it would be technically accurate, though less convenient to say 'A layer that contains Typed Text.' Why? Lets for arguments sake say that we have an image with a 'Type Layer' at the lowest index. If you rasterize the Type Layer in Photoshop or any other image editor for that matter, it will no longer be able to accept text input to change the text in it. You will still see the text but the layer now contains an image of the text and not an editable text object. Even though the layers' content has changed the rasterized image of the text will still be drawn or rendered at the lowest index in the image.
Manipulating the Layer StackThe Layers Panel is used to view and manage the stack of layers in your image. Every new image starts with a single layer. You can add or remove layers and move layers up or down to set the position of the layers in the stack.