An expansion card is an electronic circuit board that adds more functionality to a desktop computer. Expansion cards are installed into the expansion slot of a computer motherboard. They enhance the computer’s ability to perform additional functions not offered by the motherboard. Video cards and sound cards are common examples of PC expansion cards. A new video card added will enhance the three dimensional graphics processing power of a computer while a new sound card may improve a computer’s audio input.

There are alternative terms used for an expansion card. It is also known as expansion board, add-on card, interface adapter or an internal card. Generally, around one to seven expansion cards can be installed into the desktop computer system. Laptops do not use standard cards due to their small form factor. A laptop expansion card can be in the form of a removable PCMCIA card that offers additional functionalities to it.

The Altair 8800, developed in the mid 1970s, was the first microcomputer to add an expansion card bus. In 1981, IBM® launched its first IBM® PC with an XT bus, which was later replaced with a 16-bit ISA. The introduction of the PCI bus in 1991 resulted in modern forms of interface adapters that provided additional benefits beyond enhanced graphics and sound.

Most cards are inserted in PCI slots. PCI slots or “Peripheral Component Interconnect” slots are integrated circuits fitted onto the motherboard. One edge of the expansion card holding the contacts or keys is inserted into the PCI slot. This establishes an electrical contact between the motherboard and the card’s integrated circuits.

Standard interface adapters such as graphics cards and sound cards offer various added functions. Some video cards offer video capture, MPEG 2 and MPEG 4 decoding, light pen and connecting to multiple monitors. Sound cards add functions for composing music, editing audio presentations and other multimedia applications.

There are some “low-profile” cards that fit in a lower height computer framework. Some cards are used solely for external connectivity such as modem cards, storage area network (SAN) and network cards. These cards are commonly referred to as I/O cards or input/output cards. A USB expansion card is mainly used by users who need additional USB or Firewire ports.

A PC expansion card can only be inserted on computers with available expansion slots. Computers such as the Apple® iMac and other all-in-one systems do not accept expansion cards.