An Operating System (OS) is the essential software necessary so that a computer can perform useful tasks and interact with a user. Even your microwave oven has an OS, though it is obviously quite different from the one you use on your PC. There are a lot of technical details involved and the line gets fuzzy (as it always does) when you question what exactly is part of an OS and what is not. However, for our purposes, this definition suffices. By itself, an OS is not very exciting (except to OS geeks) but it is essential. The stability and security of your PC depends upon how well the OS is built.
The two dominating OSs for desktop and notebook computers are Microsoft Windows and Apple's (Mac) OS X. There is also iOS and Android for smartphones and tablets. GNU/Linux is another operating system which is used on servers (including this one), on supercomputers as well as on desktops and notebooks. Unlike most of its competitors, it's free to use and offers its users freedom in the form of FLOSS. GNU/Linux is a descendant of UNIX. Rather than just being one product, GNU/Linux encompasses a wide variety of distros In fact, Android is a GNU/Linux distro. GNU/Linux also powers devices like microwave ovens and ATMs.