First let's find out what is a Operating System! An Operating System is a software which manages the computer hardware and provides a platform for computer applications.
It is an essential part of a computer or any other electronic devices.
Many Operating Systems like Windows provide a Graphical User Interface (GUI) which gives the user an interactive control for carrying out tasks. They act as a medium between user and computer hardware.
An Operating System is very necessary because not only it gives an easy interface but also it provides security (firewall) and manage resources and memory.
With the development of technology the mainframe sizes of the O.S. is decreasing and is coming in phones , watches and now even wrist bands.
Some examples of Operating System are Windows , Android , Ubuntu , Mac OS X, iOS etc