An operating system is a specialized software package that operates something on behalf of someone. The object it operates is clearly the computer itself--the operating system operates the computer. The entity it operates on behalf of is a human user, or a community of human users. Functions of an operating system include providing a uniform interface across differing hardware, providing abstractions such as files, windows, clocks, etc., that humans prefer, and providing mechanisms for enforcing security policies. Every general-purpose computer requires some type of operating system that tells the computer how to operate and how to utilize software applications and hardware that are installed. This frees the user from the tedium of having to worry about the minute details of file reads, writes, scheduling of process threads, and the like.

Operating systems can be classified in various ways, depending for example on whether they...