Each and every personal computer or server is run by an Operating System - the low-level software which communicates with the computer hardware, including peripherals such as a mouse or a printer, and any applications that are 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 app runs within the parameters defined by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard site 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, which means that you'll be able to set up a different software environment on the very same machine.