Every single PC or server is run by an Operating System - the low-level software that interacts with the hardware equipment, including peripherals such as a mouse and a printer, and any applications that are currently 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 Operating System. On a server, each application runs within the parameters specified by the OS as well - priority, physical memory, processing time, etc. This is valid for both standard website scripts and server-side software like a media server. In case a virtual server is generated on a physical one, there can be two independent Operating Systems, named guest OS and host OS, so you can set up a different software environment on the very same machine.