Operating System (OS)
Is a group of programs that monitors how your computer system functions (The OS manages the computer's hardware, together with processor, memory, and storage)
Operating systems generally come pre-loaded on any computer you buy. Most people apply the operating system that comes with their computer, however it's possible to upgrade or even change operating systems. The three most usual operating systems for personal computers are Microsoft Windows, macOS, and Linux.
Modern operating systems apply a graphical user interface, or GUI (pronounced gooey). A GUI lets you apply your mouse to click icons, buttons, and menus, and everything is definitely shpown on the screen employing a combination of graphics and text.
For further readings go to this link http://en.wikipedia.org/wiki/List_of_operating_systems