PCI (Peripheral Component Interconnect) and Express Cards are types of expansion cards used to add functionality to a computer system. PCI cards are installed inside the computer's motherboard and are used to add features such as sound cards, network cards, and graphics cards. Express Cards, on the other hand, are smaller and are designed for use with laptops and other portable devices. They are used to add features such as USB ports, FireWire ports, and mobile broadband modems. Both PCI and Express Cards are easy to install and provide a cost-effective way to enhance the capabilities of your computer system.