Most current usage of the term "operating system" today, by both popular and professional sources, refers to all the software that is required in order for the user to manage the system and to run third-party application software for that system. That is, the common understanding includes not only the low-level "kernel" that interacts directly with the hardware, but also libraries required by applications as well as basic programs to manipulate files and configure the system.
The exact delineation between the operating system and application software is not precise, however, and is occasionally subject to controversy. For example, one of the key questions in the United States v. Microsoft antitrust trial was whether Microsoft's Internet Explorer web browser was part of its Windows operating system or if it was a separable piece of application software. As another example, the GNU/Linux naming controversy is, in part, due to disagreement about the relationship between the Linux kernel and the operating system commonly referred to as "Linux".
The lowest level of any operating system is its kernel, the first layer of software loaded into computer memory when it starts up. As the first software layer, all other software that gets loaded after it depends on this software to provide them with various common core services. These common core services include, but are not limited to: disk access, memory management, task scheduling, and access to other hardware devices. Like the term "operating system" itself, the question of what exactly should form the "kernel" is subject to some controversy—with various camps advocating "microkernels", monolithic kernels, and so on—with debates over whether things like file systems should be included in the kernel.
Today's operating systems
Command line interface (or CLI) OS's such as DOS, use only the keyboard for input. Modern OS's use a mouse for input with a graphical user interface (GUI) sometimes implemented as a shell. The appropriate OS may depend on the hardware architecture, specifically the CPU, with only Linux and BSD running on almost any CPU. Since the early 1990's the choice for personal computers has been largely limited to the Microsoft Windows family and the Unix-like family, of which Linux is becoming the major representative. Mainframe computers and embedded systems use a variety of different operating systems, many with no direct connection to Windows or Unix, but typically more similar to Unix than Windows.
Personal computers
IBM PC compatible - smaller Unix-variants, like Linux and BSD, and Microsoft Windows
Apple Macintosh - Mac OS X, Linux and BSD
Mainframes - A number of unique OS's, sometimes Linux and other Unix variants.
Embedded systems - a variety of dedicated OS's, and limited versions of Linux or other OS's
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Unix-like systems
The Unix-like family is a diverse group of operating systems, with several major sub-categories including System V, BSD, and Linux. The name "Unix" is a trademark of The Open Group which licenses it for use to any operating system that has been shown to conform to the definitions that they have cooperatively developed. The name is commonly used to refer to the large set of operating systems which resemble the original Unix.
Unix systems run on a wide variety of machine architectures. They are used heavily as server systems in business, as well as workstations in academic and engineering environments. Free software Unix variants, such as Linux and BSD, are increasingly popular. They have made inroads on the desktop market as well, particularly with "user-friendly" Linux distributions such as Ubuntu Linux.
Some Unix variants like HP's HP-UX and IBM's AIX are designed to run only on that vendor's proprietary hardware. Others, such as Solaris, can run on both proprietary hardware and on commodity x86 PCs. Apple's Mac OS X, a BSD variant derived from NeXTSTEP and FreeBSD, has replaced Apple's earlier (non-Unix) Mac OS in a small but dedicated market, in the process becoming the most popular proprietary Unix system.
Over the past several years, free Unix systems have supplanted proprietary ones in many markets. For instance, scientific modeling and computer animation were once the province of SGI's IRIX. Today, they are dominated by Linux-based or Plan 9 clusters.
The team at Bell Labs that orignally design and developed Unix, went on to develop Plan 9 and Inferno which is what Unix tried to do, but did incorrectly. It had graphics and networking built in, unlike its Unix counterparts which added it later to the design and made things messy. However, it hasn't picked up mainly because of free Unixs, and Plan 9 not orignally being free. It is now released under Lucent Public License, and starting to build up a big community of developers.
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Microsoft Windows
The Microsoft Windows family of operating systems originated as a graphical layer on top of the older MS-DOS environment for the IBM PC. Modern versions are based on the newer Windows NT core that first took shape in OS/2 and the kernel also borrows from OpenVMS. Windows runs on 32- and 64-bit Intel and AMD computers, although earlier versions also ran on the DEC Alpha, MIPS and PowerPC architectures (and there was work in progress to make it work also on the SPARC architecture).
Today Windows enjoys a near-monopoly of around 90% of the worldwide desktop market share. [1] It is also used on low-end and mid-range servers, supporting applications such as web servers and database servers. In recent years, Microsoft has spent significant marketing and R&D money to demonstrate that Windows is capable of running any enterprise application (see the TPC article).
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Other operating systems
Mainframe operating systems, such as IBM's z/OS, and embedded operating systems such as QNX, eCos, and PalmOS, are usually unrelated to Unix and Windows, except Windows CE, Windows NT Embedded 4.0 and Windows XP Embedded which are related to Windows and several *BSDs and Linux distributions tailored for the requirements of an embedded system. OpenVMS from Hewlett-Packard (formerly DEC), is still under active development.
Older operating systems which are still used in niche markets include the Windows-like OS/2 from IBM; Mac OS, the non-Unix precursor to Apple's Mac OS X; RISC OS, which is specifically designed to run on ARM processor architectures; and AmigaOS, the first graphical user interface (GUI) based operating system with advanced multimedia capabilities available to the general public.
Research and development of new kinds of operating systems is an active subfield of computer science. Microsoft Singularity is a research project to develop an operating system with better memory protection.