Bit depth

also called pixel depth or color depth--measures how much color
information is available to display or print each pixel in an image. Greater bit depth
(more bits of information per pixel) means more available colors and more accurate
color representation in the digital image. For example, a pixel with a bit depth of 1
has two possible values: black and white. A pixel with a bit depth of 8 has 28, or
256, possible values. And a pixel with a bit depth of 24 has 224, or roughly
16 million, possible values. Common values for bit depth range from 1 to 64 bits
per pixel.

In most cases, Lab, RGB, grayscale, and CMYK images contain 8 bits of data per
color channel. This translates to a 24-bit Lab bit depth (8 bits x 3 channels); a
24-bit RGB bit depth (8 bits x 3 channels); an 8-bit grayscale bit depth (8 bits x
1 channel); and a 32-bit CMYK bit depth (8 bits x 4 channels). Photoshop can also
read and import Lab, RGB, CMYK, and grayscale images that contain 16 bits of
data per color channel.