An Operating System is complex series of software components that run together to simulate an environment that either acts as an abstraction layer between the hardware of the device, and dependant software that runs under the operating system, or passes through calls to the hardware directly from the software. The operating system serves as a software platform to other programs, commonly termed applications.
An operating system ensures that the system hardware is divided up correctly for the software that require it, while ensuring that no conflicts occur, either between differing pieces of software, or differing pieces of hardware.
The operating system is also responsible for translating input entered into the device, and translating that input into readable instructions that are then interpreted to output(s) attached to the system.
The range of operating systems is vast, and many examples litter the history books of system architectures long-past; CP/M, AIX, MS-DOS, UNIX, OS/2, Mac OS, etc, as well as many more recent additions, such as Microsoft Windows, Mac OS X, and the distributions of Linux and BSD UNIX.