Cisco IOS (Internetwork Operating System) is a proprietary operating system used on Cisco routers and switches. It provides the necessary software to manage network devices, enabling them to route data, enforce security policies, and support various network protocols. The system features a command-line interface (CLI) that allows network administrators to configure and monitor devices efficiently. Cisco IOS supports a wide range of networking functions, including routing, switching, and network management, making it essential for building and maintaining robust computer networks.