Every PC or server is run by an Operating System - the low-level program which interacts with the hardware, including peripheral devices such as a mouse or a printer, as well as any apps 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, and so on. This is valid for both standard website scripts and server-side software such as a media server. In case a virtual server is created on a physical one, there can be two independent Operating Systems, referred to as guest OS and host OS, so you'll be able to set up a different software environment on the same machine.