This document provides an overview of operating systems including definitions, functions, evolution, and types. It defines an operating system as software that enables computer hardware to function and applications to run. Key points covered include: the structure of OS with hardware, software, system programs and applications; evolution from batch processing to timesharing and personal computing; main functions like memory management and I/O control; and types of OS such as single-user and multi-user systems. Examples of operating systems are given throughout like Windows, Linux, and Unix.