CMYK Printing
Four color process is a system where a color image is separated into 4 different color values by the use of filters and screens (usually done digitally). The result is a color separation of 4 images that when transferred to printing plates and printed on a printing press with the colored inks cyan (blue), magenta (Red), yellow and black, reproduces the original color image.
Unlike inkjet printers or color photocopiers, full color (C-M-Y-K) printing provides accurate, detailed, brilliant reproduction of your originals at a low cost-per-sheet. Look at any magazine cover (Time, People, National Geographic) for an example of 4-color process printing.
*This is the kind of printing found at "commercial printers", not your local quick-printer shop.