CMYK - Full Colour Printing
Full Colour printing (Process Printing) is a printing is a technique widely used today. This process allows you to produce nearly every colour under the rainbow.
The process invloves separatating files to be printed in to four color components: cyan, magenta, yellow, and key, also called black. The color separation is known as color screening, this is a generally a digital process. A separate print plate will be made for each color.
Each plate is created as a halftone, meaning that the color is separated into a series of tiny dots. In areas where there is very little colour, the dots will be small and spaced well apart. In areas of colour strong, the dots will be larger, and may have no space in between them.
Each plate is printed separately and overlaid over the previously printed ink. When all four plates have been printed, the result is a color image. The small dots on each individual plate will come together to create a complete color image, although they can still be seen with the aid of a magnifying glass. If the plates are not properly aligned during the printing process, the result is a misregistered image which is blurry and illegible. See Stochatic Screening
You'll need a Pantone swatchbook to accurately pick process colours. Keep in mind that not all spot colours translate well to process colours.