*An operating system is a program that acts as an interface between interface between the user and the computer. Operating system has earned the reputation for being the most critical software in a computer system. Operating system primarily provides a convenient interface to its users and at the same time manages the computer’s resources processor, memory, and I/O devices. In a nutshell operating system can be defined as a resource manager. A general organization of an operating system is as follows-accounting, input/output, command interpreter, data management, program development tools, time-sharing, security, communication. Without an operating system the most powerful computer in the world would be useless. No matter how powerful and elegant your programs are, they can’t function without the assistance of an operating system. *The operating system is characterized in three types-single user operating system, multi user operating system and multiprocessor operating system. 1>Single user operating system is popularly known as personal computer operating system. Their job is to provide a good interface to a single user. The two popular operating systems under this category are DOS and Windows. DOS is an example for a single user operating system that is single user with no multitasking. Multitasking means running more than one program concurrently. Windows is an example for a single user operating system with multitasking capability. There are various versions of operating system like, window 95, window 98 and window XP These operating systems are widely used for small applications like word processing, Internet access, managing small databases etc. 2>Multi user system is that operating which can serve for more than one user at a time, are known as multi user operating systems. Each user can run his own program. The operating system allots a quantum of time for each user for processing his tasks. The most popular multi user operating system is UNIX Operating system. 3>Multi processor operating system is those in which some computer systems involve more than one CPU. Depending on precisely how these CPU’s are connected and what is shared, these computers are called parallel computers or multiprocessor computers. They need a special type of operating system to manage the additional resources.