An operating system (OS) is an interface between hardware and user which is responsible for the management and coordination of activities and the sharing of the resources of the computer that acts as a host for computing applications run on the machine. As a host, one of the purposes of an operating system is to handle the details of the operation of the hardware. This relieves application programs from having to manage these details and makes it easier to write applications. Almost all computers (including handheld computers, desktop computers, supercomputers,video game consoles) as well as some robots,domestic appliances (dishwashers, washing machines), and portable media players use an operating system of some type. Some of the oldest models may, however, use an embedded operating system that may be contained on a data storage device.

Operating systems offer a number of services to application programs and users. Applications access these services through application programming interfaces (APIs) or system calls. By invoking these interfaces, the application can request a service from the operating system, pass parameters, and receive the results of the operation. Users may also interact with the operating system with some kind of software user interface (SUI) like typing commands by using command line interface (CLI) or using a graphical user interface (GUI, commonly pronounced “gooey”). For hand-held and desktop computers, the user interface is generally onsidered part of the operating system. On large multi-user systems like Unix and Unix-like systems, the user interface is generally implemented as an application program that runs outside the operating system.(Whether the user interface should be included as part of the operating system is a point of contention.)

Common contemporary operating systems include BSD, Darwin (Mac OS X), Linux, SunOS(Solaris/OpenSolaris), and Windows NT (XP/Vista/7). While servers generally run or some Unix-like operating system, embedded system markets are split amongst several operating systems, although the Microsoft Windows line of operating systems has almost 90% of the client PC market.

Through the 1950s, many major features were pioneered in the field of operating systems. The development of the IBM System/360 produced a family of mainframe computers available in widely differing capacities and price points, for which a single operating system OS/360 was planned (rather than developing ad-hoc programs for every individual model). This concept of a single OS spanning an entire product line was crucial for the success of System/360 and, in fact, IBM`s current mainframe operating systems are distant descendants of this original system; applications written for the OS/360 can still be run on modern machines. In the mid-70's, the MVS, the descendant of OS/360 offered the first implementation of using RAM as a transparent cache for data.

OS/360 also pioneered a number of concepts that, in some cases, are still not seen outside of the mainframe arena. For instance, in OS/360, when a program is started, the operating system keeps track of all of the system resources that are used including storage, locks, data files, and so on. When the process is terminated for any reason, all of these resources are re-claimed by the operating system. An alternative CP-67 system started a whole line of operating systems focused on the concept of virtual machines.

Control Data Corporation developed the SCOPE operating system in the 1960s, for batch processing. In cooperation with the University of Minnesota, the KRONOS and later the NOS operating systems were developed during the 1970s, which supported simultaneous batch and timesharing use. Like many commercial timesharing systems, its interface was an extension of the Dartmouth BASIC operating systems, one of the pioneering efforts in timesharing and programming languages.

In the late 1970s, Control Data and the University of Illinois developed the PLATO operating system, which used plasma panel displays and long-distance time sharing networks. Plato was remarkably innovative for its time, featuring real-time chat, and multi-user graphical games. Burroughs Corporation introduced the B5000 in 1961 with the MCP, (Master Control Program) operating system. The B5000 was a stack machine designed to exclusively support high-level languages with no machine language or assembler, and indeed the MCP was the first OS to be written exclusively in a high-level language – ESPOL, a dialect of ALGOL. MCP also introduced many other ground-breaking innovations, such as being the first commercial implementation of virtual memory. During development of the AS400, IBM made an approach to Burroughs to licence MCP to run on the AS400 hardware. This proposal was declined by Burroughs management to protect its existing hardware production. MCP is still in use today in the Unisys ClearPath/MCP line of computers.

UNIVAC, the first commercial computer manufacturer, produced a series of EXEC operating systems. Like all early main-frame systems, this was a batch-oriented system that managed magnetic drums, disks, card readers and line printers. In the 1970s, UNIVAC produced the Real-Time Basic (RTB) system to support large-scale time sharing, also patterned after the Dartmouth BASIC system.

General Electric and MIT developed General Electric Comprehensive Operating Supervisor (GECOS), which introduced the concept of ringed security privilege levels. After acquisition by Honeywell it was renamed to General Comprehensive Operating System (GCOS).

Digital Equipment Corporation developed many operating systems for its various computer lines, including TOPS-10 and TOPS-20 time sharing systems for the 36-bit PDP-10 class systems. Prior to the widespread use of UNIX, TOPS-10 was a particularly popular system in universities, and in the early ARPANET community.

In the late 1960s through the late 1970s, several hardware capabilities evolved that allowed similar or ported software to run on more than one system. Early systems had utilized microprogramming to implement features on their systems in order to permit different underlying architecture to appear to be the same as others in a series. In fact most 360's after the 360/40 (except the 360/165 and 360/168) were microprogrammed implementations. But soon other means of achieving application compatibility were proven to be more significant.

The enormous investment in software for these systems made since 1960s caused most of the original computer manufacturers to continue to develop compatible operating systems along with the hardware. The notable supported mainframe operating systems include.