From LQWiki
An operating system (OS) is the fundamental software that controls your computer at the most basic level, creating services to applications such as drawing to the screen, playing sounds, loading files and connecting to other computers on the network. Generally speaking, it is the most basic software that communicates with the physical hardware of your computer. Computers, cell phones, PDAs, POS devices, etc. are all examples of computing devices that are likely to have an operating system installed.
At the core of an operating system is a unit known as the kernel. The kernel can be designed in different ways, such a monolithic kernel or microkernel (or other philosophy). Monolithic kernels aim to provide many basic and complex services, while microkernels provide the most essential services and have external software to provide more complex services.
Some OS's have a modular monolithic kernel (like GNU/Linux, BeOS, and MS Windows98), others have a microkernel (like GNU/HURD, MS Windows2000 and Mach), and still others employ more exotic designs (ie. exokernels). OS's are further distinguished by whether or not they are real-time OS's.
An older term, "network operating system" (ex. Novell NetWare) has fallen out of use. It used to mean an OS specifically designed to work in a networked environment. These days, most popular operating systems do fine on a network.
Applications written for one operating system cannot usually be made to run on another without a huge amount of work, because of the different designs and because the API implementations (the services the programs use) don't exist on the competing platform. Free Software bypasses such difficulties because the source code that applications depend on (the libraries and kernel) is available to anyone who wants to make the effort of porting it to another platform. Because of this openness, it is relatively easy to port a paint program such as GIMP to Windows, while porting (or even emulating) a Windows-native paint program such as Paint Shop Pro is much more difficult.
Operating systems are often compliant to one or more standards, such as POSIX or the Single Unix Specification. There is some overlap here; for example, Windows NT is hardly a Unix system, but it is partially POSIX-compliant.
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