An operating system is a big word to say "the software that runs your computer." Without this software, your computer is completely useless and unable to perform any task.
It is very useful to know what operating system you use for this information is what let you know which software is compatible with your PC.
Also note that most of the technicians who can charge exorbitant fees to "setup your computer" only install the operating system for you. One thing you could probably easily do for yourself. (see install an operating system
Here are the most popular operating systems :