CompTIA A+ Core 2 (220-1002) Study Guide

Chapter 01: Operating Systems
An Operating System (OS) is the fundamental software running on a computing device. It is a system software that manages computer software and hardware resources and provides shared services for computer programs. Each computer must have at least one OS in order to run other programs. The two most common operating systems are Windows and Mac OS; however Linux is the most secure family of operating systems.
An OS is a program that, after being initially loaded into the computer by a boot program, manages all application programs on a computer. The application programs make requests to the OS for services, via a defined Application-Programming Interface (API). Users can also directly interact with the OS through a user interface such as a Graphical User Interface (GUI) or a command line interface.