An operating system is software that manages computer hardware and resources, allowing users to interact with the computer. There are several types of operating systems including single-user, multi-tasking, real-time, embedded, desktop, and server operating systems. Popular desktop operating systems include Windows and MacOS, while mobile operating systems include Android and iOS. The document provides brief histories of Windows, MacOS, Android and iOS, noting the first versions and some updates that introduced new features to each operating system over time.