Every PC or server is run by an Operating System - the low-level program which communicates with the computer hardware, including peripherals such as a mouse or a printer, along with any apps that are currently installed on the 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 server, each application runs within the parameters defined by the OS as well - priority, physical memory, processing time, etcetera. This goes for both standard website scripts and server-side software like a media server. If a virtual server is created on a physical one, there can be two separate Operating Systems, called guest OS and host OS, so you can set up a different software environment on a single machine.