Each personal computer or web server is run by an Operating System - the low-level software that communicates with the hardware, including peripheral devices such as a mouse or a printer, and any applications that are 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, every single application runs within the parameters defined by the OS as well - priority, physical memory, processing time, etcetera. 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 different Operating Systems, called guest OS and host OS, so that you'll be able to set up a different software environment on the same machine.