Process color is the method of printing full spectrum color using four primary inks.
Process Color is commonly referred to as "CMYK" because the four primary inks are Cyan, Magenta, Yellow, and Black. Process inks have a transparent base, so when two or more overlap each other they combine to create a new hue.
Black is the Key Color in process printing, which is why it is the K in CMYK. Using "K" instead of "B" also prevents confusion between black and blue.
The CMYK color model is used in commercial printing as well as with ink-jet printers.