Operating system is a bridge between system and user. An operating system (OS) is a software program that manages the hardware and software resources of a computer. The OS performs basic tasks, such as controlling and allocating memory, prioritizing
the processing of instructions, controlling input and output devices, facilitating networking, and managing files. It is difficult to present a complete as well as deep account of operating systems developed till date. So, this paper tries to overview only a subset of the available operating systems and its different categories. OS are being developed by a large number of academic and commercial organizations for the last several decades. This paper, therefore, concentrates on the different categories of OS with special emphasis to those that had deep impact on the evolution process. The aim of this paper is to provide a brief timely commentary on the different categories important operating systems available today.
