As DaClyde said, cards are pinted using a four color process. The image of the card is split into four 'channels'. This four color scheme for creating all of the colors in the image is known as CMYK, which stands for Cyan, Magenta, Yellow, & Black. So every sheet of cards is printed using four press plates, one for each color ink. Some card companies have then cut up the press plates into sections for each card and insert them in packs.
-Tom
Ted Musgrave card collection 98.9% Complete: Cards Known: 1013, Cards Owned: 1002
I prefer the company of people who disagree with me for the right reasons over the company of those who agree with me for the wrong reasons.