An expansion card is a printed circuit board that can be installed in computer to add functionality to it. For example, a user may add a new graphics card to his computer to give it more 3D graphics processing power. An audio engineer may add a professional sound card to his machine to increase the computer's audio input and output connections. Users that need more Firewire or USB ports can add Firewire or USB expansion cards, which provide additional connections.
Most expansion cards are installed in PCI slots. This includes variations of PCI, such as PCI-X and PCI Express. Graphics cards may also be installed in an AGP slot, which is designed specifically for video cards. Since expansion cards require open slots, they can only be installed in computers that have available expansion slots. Therefore, computers like the Apple iMac and other all-in-one machines cannot accept expansion cards. Computer towers, however, often have two or three open expansion slots, and can accept multiple cards.