bMighty SMB TechEncyclopedia

A printed circuit board that plugs into an expansion slot on the motherboard and extends the computer's capability to control a peripheral device. Also called a "card," "interface card," "adapter" or "controller," all the printed circuit boards that plug into a computer's bus are technically expansion boards, because they expand the computer's capability. Typical examples are the display adapter, network adapter (NIC) and sound card; however, all of these circuits may be contained in chips on the motherboard (see PC chipset).

They Used to Be the Norm
In earlier PCs, all peripheral controllers were housed on expansion boards, including the disk controller, serial and parallel ports, sound and display. Today, all the peripheral control may be included in the chipset (see PC chipset). In such cases, users still have the option of plugging in their own controllers. For example, in order to enhance video game performance, you could purchase a faster display adapter, plug it into the AGP or PCI slot and disable the built-in display circuit. See motherboard. See also bus extender.