A computer by itself is unusable. In order to use a computer it has to be running an operating system. An operating system is software which runs all the time and manages the computer hardware and basic software interactions. Your computer is running an operating system of some kind. Most people are using a Microsoft Windows based computer, which means that their operating system is some version of Windows. Those people who are using Apple Macintosh computers are most likely using the OSX operating system. Another popular PC operating system is Linux.