Separate "drawing boards" or "canvasses" in a drawing, CAD or image editing program that are overlaid one on top of the other to complete the picture. Layers provide greater control over the illustration or image and offer different functions depending on the application. For the differences in graphics applications, see graphics
Drawing Programs (Vector Graphics)
Computer-aided design (CAD) programs were the first software to provide layers, and advanced illustration programs followed suit. Graphic objects can be placed in separate layers on top of or below other layers, enabling related objects to be worked on as a group without affecting the rest of the drawing. See graphics
Imaging Programs (Bitmapped Graphics)
Layers are available only in advanced imaging programs such as Photoshop, otherwise all editing is done on a single plane. In single-layer paint and image editing software, when a graphic object is added, it replaces the pixels that occupied that same space. Consequently, if the object is removed, it leaves blank space in its stead.
However, in a layered imaging program, objects are placed in separate layers and can be freely moved under and over each other. Objects can always be added, and any object can be removed or changed without affecting the others. The final result is the composite of all layers, which can be "flattened" into one layer for conversion into a JPEG, BMP, GIF or other bitmapped file format. The original image can be maintained in its layered format for future modifications, such as in Photoshop's proprietary PSD format.
Layers in Photoshop
The bitmap image above is built in 23 layers plus a background layer, enabling the individual elements to be moved independently of the others. The Photoshop layer window (below) shows the layers. The red arrow is pointing to the view option (the "eye") in the chip shadow layer. Clicking the eye toggles the layer between visible and invisible.