8-bit images have pixels with interger values between 0 and 255. Such images are commonly known as Monochrome or Greyscale. In color images each color channel, Red, Green and Blue, commonly has 8-bit color depth. This may be refered to as 24-bit (8 * 3 bit) color or 32 bit (8 * 4 bit) color if transparency information is included.
8-bit color depth is common for consumer electronics, while 12-bit or higher being used for specialist application. 8-bit per channel reproduction is sufficient for the human eye to preceve 'full' color.