Operating System (OS)

An operating system (OS) is software, consisting of programs and data, that runs on computers, manages computer hardware resources, and provides common services for execution of various application software.
)The most important program that runs on a computer. Every general-purpose computer must have an operating system to run other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers.
For large systems, the operating system has even greater responsibilities and powers. It is like a traffic cop -- it makes sure that different programs and users running at the same time do not interfere with each other. The operating system is also responsible for security, ensuring that unauthorized users do not access the system.
Operating systems can be classified as follows:

multi-user : Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users.

multiprocessing : Supports running a program on more than one CPU.

multitasking : Allows more than one program to run concurrently.

multithreading : Allows different parts of a single program to run concurrently.

real time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time.

Operating systems provide a software platform on top of which other programs, called application programs, can run. The application programs must be written to run on top of a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run. For PCs, the most popular operating systems are DOS, OS/2, and Windows, but others are available, such as Linux.
As a user, you normally interact with the operating system through a set of commands. For example, the DOS operating system contains commands such as COPY and RENAME for copying files and changing the names of files, respectively. The commands are accepted and executed by a part of the operating system called the command processor or command line interpreter. Graphical user interfaces allow you to enter commands by pointing and clicking at objects that appear on the screen.

Network operating system (NOS)

Networking operating system (NOS), also referred to as the Dialoguer, is the software that runs on a server and enables the server to manage data, users, groups, security, applications, and other networking functions.The network operating system is designed to allow shared file and printer access among multiple computers in a network, typically a local area network (LAN), a private network or to other networks.

Characteristics 

Network Operating Systems are based on a client/server architecture in which a server enables multiple clients to share resources.

The Network Operating System can also do the following:

• Centrally manage network resources, such as programs, data and devices.
• Secure access to a network.
• Allow remote users to connect to a network.
• Allow users to connect to other networks like the Internet.
• Back up data and make sure it's always available.
• Allow for simple additions of clients and resources.
• Monitor the status and functionality of network elements.
• Distribute programs and software updates to clients.
• Ensure efficient use of a server's capabilities.

Multitasking

Multitasking, in an operating system, is allowing a user to perform more than one computer task (such as the operation of an application program) at a time. The operating system is able to keep track of where you are in these tasks and go from one to the other without losing information.
Microsoft Windows 2000, IBM's OS/390, and Linux are examples of operating systems that can do multitasking (almost all of today's operating systems can). When you open your Web browser and then open Word at the same time, you are causing the operating system to do multitasking.

MS-DOS

• MS-DOS (; short for Microsoft Disk Operating System) is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating systems offering a graphical user interface (GUI), in particular by various generations of the Microsoft Windows operating system.
• MS-DOS grew from a 1981 request by IBM for an operating system for its IBM PC range of personal computers. Microsoft quickly bought the rights to QDOS (Quick and Dirty Operating System), also known as 86-DOS, from Seattle Computer Products, and began work on modifying it to meet IBM's specification. The first edition, MS-DOS 1.0, was launched in 1982.
• MS-DOS (Microsoft Disk Operating System) was the Microsoft-marketed version of the first widely-installed operating system in personal computers. It was essentially the same operating system that Bill Gate's young company developed for IBM as Personal Computer .

Macintosh OS

• Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. (formerly Apple Computer, Inc.) for their Macintosh line of computer systems. 
• The original form of what Apple would later name the "Mac OS" was the integral and unnamed system software first introduced in 1984 with the original Macintosh, usually referred to simply as the System software.

LINUX

• is a computer operating system which is based on free and open source software.
• Linux can be installed on a wide variety of computer hardware, ranging from mobile phones, tablet computers, routers and video game consoles, to desktop computers, mainframes and supercomputers. Linux is a leading server operating system, and runs the 10 fastest supercomputers in the world.

Graphical user interface(GUI)

• In computing, a graphical user interface (GUI, sometimes pronounced gooey) is a type of user interface that allows users to interact with electronic devices with images rather than text commands.
• GUI's can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and office equipment.
• A GUI represents the information and actions available to a user through graphical icons and visual indicators such as secondary notation, as opposed to text-based interfaces, typed command labels or text navigation.