What are the purpose of operating system and its functions?

The purpose of an operating system  is to manage computer memory, processes and the operation of all hardware and software ,also to provide a platform on which a user can execute programs in a convenient and efficient manner.
An operating system has three main functions:

(1) Is to manage the computer's resources, such as the central processing unit, memory, disk drives, and printers, (2)  to establish a user interface, and (3)to execute and provide services for applications software.