Controller Cards are expansion cards used to add additional functionality to a computer, typically in the form of extra ports, such as USB, SATA, or RAID. These cards are installed into a motherboard's expansion slots to expand the system's connectivity options or enhance its capabilities. Controller cards are often used in server systems, gaming setups, and workstations that require additional storage or peripheral connections. Shopping for controller cards online offers users a variety of options to improve system performance and connectivity.