An operating system is a main program on computer, that governs all other applications. It allows you to use browsers, play games, print documents, launch your favorite program.

 Most common Operation Systems:

• Windows (desktops mostly) 
• Mac OS (laptops mostly) 
• Android (cell phones) 
• Linux (hard-code users & programmers) 

How Operation System work. Basics 

Computer CPU knows how to execute commands. But it needs someone to tell him what command to execute. It is the job of Operation System (OS). Whenever you power your computer, it reads first bytes on your Disk Drive, where it expects to see commands that will start Operation System. 

Components of Operation System

• OS Kernel
• main part of OS, it facilitates interactions between hardware and software components and performs task scheduling 
• Drivers - small programs that are responsible for interaction with different hardware: keyboard, printer, usb drive etc 
• User Interface - responsible for displaying all those windows, buttons and so on. Based on DirectX/OpenGL from once side, and on some UI Framework from other side (WPF, Windows Forms, WinAPI, HTML, etc) 

Please use below materials for deep dive into the topic