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. Operating systems are being developed by a large number of academic and commercial organizations for the last several decades.