I/O cards (Input/Output cards) are a type of expansion board used to transfer data between computer systems and other electronic devices. These cards plug into the computer's motherboard and provide additional input/output connections.

I/O cards can work with a wide variety of devices. For example, an I/O board can be connected to a printer or scanner by connecting to the computer's parallel port. They also offer a variety of connectivity options such as serial ports, Ethernet ports, USB ports and audio ports.

However, nowadays the use of I/O cards is declining as many devices are already able to connect directly to the computer via other means such as wireless or Bluetooth technology. However, some specialized devices are still connected using I/O boards and are still required for some industrial applications.