An Operating System ("OS") is a software layer on which applications and program are executed. It is hardware-dependant and factorizes many functionalities to make them simply accessible to programs, such as file management. Often, it also offers to have multiple programs executed at the same time by slicing their processor access. You can find more information on the dedicated wikipedia page.
As the most known OSes, we can cite:
- Windows XP, Windows Vista, Windows Seven, ...
- MacOS X
- GNU/Linux based OSes like Debian or Ubuntu.