Operating System (OS) is normally a hugely complex program that's typically installed before any other program and serves as a platform for running other programs as well as managing resources (CPU usage, RAM, files, network, ...) and offering services and interfaces for humans and programs.
There is a nice CC0 wiki for OS development at https://wiki.osdev.org/.
From programmer's point of view a serious OS is one of the most difficult pieces of software one can pursue to develop. The task involves an enormous amount of low-level programming, development of own tools from scratch and requires deep and detailed knowledge of all components of a computer, of established standards as well as many theoretical subjects such as compiler design.
An OS, as a software, consists of two main parts:
For example in GNU/Linux, Linux is the kernel and GNU is the userland.
Below are some of the most notable OSes.
All content available under CC0 1.0 (public domain). Send comments and corrections to drummyfish at disroot dot org.