You can think of the grayscale as the brightness dimension of the HSB scheme (or the axis of the HSB cone)—with saturation held to zero, and hue therefore meaningless. The grayscale is used by photographers, and it is also useful in many documents where variations in gray can be used in place of costly color printing.
Our eyes can actually distinguish about eleven different shades of gray (see the “Zone system” page.)
The only possible confusion with a gray scale—always described in percent—is to distinguish between the brightness scale (black = 0, white = 100) and the complimentary scale (100 minus brightness) that printers and photographers use to describe it. In Photoshop however, it is displayed as an 8 bit number regardless of the real bit depth (16 bit is represented as if it were 8 bit.) Therefore black = 0, white = 255, & middle gray = 127. It is a linear scale.