A card, or printed circuit board, also known as a board, is a piece of plastic with chips attached to it. Chips are the tiny little circuits that run the computer. You buy a card and stick it inside the computer box. You can get accelerator cards (boards) that make your computer run faster, video cards that give your computer more graphic capability, and clock cards and printer cards or whole computers on a card. They range in price depending on what they do, who makes them, and so on. For instance, an accelerator card can cost $300-$1500.
Many cards you just order through the mail and then depending on what kind of computer you have you can open the back of the machine and stick 'em in, which I find to be a very frightening experience. Actually, once you take a few simple precautions, convince yourself (which sometimes you can only do if you have no choice) that you won't get electrocuted and you won't permanently destroy your computer, it can be a very satisfying, rewarding, and empowering experience to install a card yourself. I know.