Each PC or server is run by an Operating System - the low-level software that communicates with the computer hardware, including peripheral devices such as a mouse or a printer, together with any apps that are currently installed on your 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 OS. 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. In case 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 you'll be able to set up a different software environment on a single machine.