In general, an Operating System is some kind of software that is installed on a computer, laptop, tablet or phone in order for someone to be able to use it. The software is installed on the hardware and provides a number of default programs for usage as well as the possibility to install new programs, applications and the like. Without the OS, a normal user is unable to use the device in question.
What the Operating System does is transport the input of the user (e.g. what’s typed on a keyboard or a mouse click) to the computer who processes it. Likewise, it provides the output of these actions for the user to see on the computer monitor or decide screen. Furthermore the OS controls all external devices like hard drives, printers, scanners and the like and keeps track of the file directory and storage.