An operating system (OS) is a complex piece of software responsible for managing a computers hardware, internal and external devices, memory, software and services. The term can both be used to describe a kernel that provides only very basic functionality and complete systems with graphical interfaces and complex applications.
The Linux kernel, written by Linus Torvalds, is considered to en an operating system yet it is just a system kernel that does not provide any graphical user-interface or usable applications. Variants of The GNU Operating System such as Gentoo Linux, HamoniKR, Linux Mint and Manjaro Linux are also considered to be operating systems in their own right even though they consist of a Linux kernel and many other components like the X display server, complex desktop environments like Xfce and KDE Plasma and huge and complex applications like LibreOffice.
Microsoft Windows 10, a non-free operating system from American computer giant Microsoft, has a kernel, a display server, an app-store, many built-in applications and much more. It contains much more than what was considered to be an operating system in the 1980s when simple program capable of providing resource management and a simple command prompt were considered to be an operating system.