Each PC or server is run by an Operating System - the low-level software which communicates with the hardware equipment, including peripheral devices such as a mouse or a printer, as well as any applications that are currently 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 server, each and every app 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 such as a media server. If a virtual server is set up on a physical one, there can be two independent Operating Systems, called guest OS and host OS, which means that you'll be able to set up a different software environment on a single machine.