An operating system is a software that enables all other programs and hardware to function properly. It provides a user interface, runs programs, manages devices and storage. The OS acts as an interface between users, programs, and hardware. It allows for parallel activity to avoid wasted cycles and provides protection for users. There are different types of operating systems including real-time, single-user/single-tasking, single-user/multitasking, and multi-user/multitasking operating systems.