Every single PC or web server is run by an Operating System - the low-level software which communicates with the hardware, including peripheral devices such as a mouse or a printer, along with any applications that are installed on the system. Any program input through 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 web server, each and every app runs within the parameters set by the OS as well - priority, physical memory, processing time, and so on. This is valid for both standard site scripts and server-side software such as a media server. When a virtual server is generated on a physical one, there can be two different Operating Systems, referred to as guest OS and host OS, so that you will be able to set up a different software environment on a single machine.