Each notebook or desktop or web server is run by an Operating System - the low-level software which communicates with the computer hardware, including peripheral devices such as a mouse or a printer, together with any applications that are installed on your system. Any program input using a command line or a Graphical User Interface (GUI) is processed by sending an Application Program Interface (API) request to the OS. On a web server, any app runs within the parameters defined by the OS as well - priority, physical memory, processing time, etc. This goes for both standard website scripts and server-side software like a media server. In case a virtual server is created on a physical one, there can be two different Operating Systems, called guest OS and host OS, so that you can set up a different software environment on a single machine.