Let's begin with a definition: What is an Operating System? To put it in the simplest terms,
it's the EXECUTIVE MANAGER, the part of the computing system that manages all of the hardware and all of the software. To be specific, it controls every file, every device, every section of main memory, and every nanosecond of processing time. It controls who can use the system and how. In short, it's the BOSS.
Therefore, when the user sends a command, the operating system must make sure that the command is executed or, if it's not executed, must arrange for the user to get a message explaining the error. This doesn't necessarily mean that the operating system executes the command or sends the error message-but it does control the parts of the system that do.
A computer system consists of software(programs) and hardware(the physical machine and its electronic components). Operating system software falls in a broad category of software. Other categories can include utility software and application software, (programs such as word processors or media players that perform specific tasks for users). The next two sections describe the components of operating system software, and the machine hardware on which it runs.
Reference: Flynn, Ida M. and McHoes, A.M., Understanding Operating Systems(Fourth Edition) by Course Technology 2007
