OS (operating system)
A computer by itself is essentially dumb bits of wire and silicon. An operating system knows how to talk to this hardware and can manage a computer’s functions, such as allocating memory, scheduling tasks, accessing disk drives, and supplying a user interface.
Without an operating system, software developers would have to write programs that directly accessed hardware – essentially reinventing the wheel with every new program.
With an operating system, such as Windows NT or Mac OS 8, developers can write to a common set of programming interfaces called APIs and let the operating system do the dirty work of talking to the hardware.