What is an Operating System (O.S) for a Computer System

An operating system (OS) is the software that manages the sharing of the resources of a computer and provides programmers with an interface used to access those resources. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the system. At the foundation of all system software, an operating system performs basic tasks such as controlling and allocating memory, prioritizing system requests, controlling input and output devices, facilitating Computer networking and managing files. Most operating systems come with an application that provides an interface for managing the operating system. The operating system forms a platform for other software.
Common contemporary desktop OSes are Linux, Mac OS X, Microsoft Windows and Solaris. Windows is most popular on desktops, Linux is most popular in servers environments. However many Linux distributions are gaining in popularity on desktop personal computers. Linux, Mac OS X and MS Windows all have server and personal variants. With the exception of Microsoft Windows, the design of each of the aforementioned OSs were inspired by, or directly inherited from, the Unix Operating System. Unix was developed at Bell Labs beginning in the late 1960s and spawned the development of numerous free and proprietary operating systems.

source:wikipedia