What is an operating system? What are the main functions of a modern general purpose operating system?

An operating system is a software that allows a user to run other applications on a computing device. The applications generally are not designed to interface directly with hardware, almost all of the applications are written to run on an operating system, in order to become independent of the hardware design.

The main function of an operating system are:

manage the hardware resources of a computer: central processing unit, memory, disk drives, and printers. establish a user interface to facilitate the interaction human-computer. execute and provide services for software applications.