A computer add-on card is an electronic card/board that is used to add extra functionality to a computer. It is inserted into an expansion slot on the motherboard of a computer. Add-on cards contain edge connectors that are used to create an electronic link between motherboard and card, thus enabling these two to communicate. Add-on cards are also known as expansion cards or interface cards. Many different classes of add-on card are available, including sound cards, video graphics cards, network cards and so on. All add-on cards are used to enhance the quality of their specific function. For example, video graphics cards are used to enhance the video quality on a computer. The basic purpose of add-on cards is to enhance the existing abilities of the motherboard. The adoption of add-on cards occurred rapidly in the computing world because of the ability for users to customize performance.

Giving a computer additional capabilities requires fitting the computer with an add-on card, also called an expansion card, interface card or just plain card. Expansion controller cards have sockets that connect to conductors carrying data within the system. The connector you use depends on the card and computer. Cards provide communications interfaces, extra memory, video sound I/O capabilities, system interfaces for discs or tape backup devices and additional processors in general. Controller cards are an excellent example of an add-on card because the card acts as an interface for the motherboard, and other computer components like HDD, optical drives and keyboards. Add-on cards are suitable for use with printers and other devices in office environments.