PC Add-On Cards are devices used to expand the capabilities of a computer system. They are available in various forms such as Communication Cards, RS232, RS485, IEEE, PCIe/PCI max 128 Serial and PCIe max 128 Serial. These cards allow for a wide range of communication and data transfer options, making them ideal for a variety of applications. They are also highly reliable and cost-effective, making them a great choice for expanding the capabilities of any PC.