This document discusses computer operating systems. It defines an operating system as a collection of software that manages computer hardware resources and provides common services for computer programs. It notes that operating systems are necessary because without them, every program would need full hardware specifications and drivers to run correctly. The document then outlines the history of major operating systems from the 1950s to present day, including UNIX, Mac OS, Windows, Linux, Android and iOS. It also discusses the components of an operating system and how they have evolved from early command line interfaces to modern graphical user interfaces.