Every personal computer or server is run by an Operating System - the low-level program which interacts with the hardware equipment, including peripheral devices such as a mouse and a printer, as well as any applications that are present 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 set by the OS as well - priority, physical memory, processing time, and so on. This is valid for both standard site scripts and server-side software like a media server. When a virtual server is created on a physical one, there can be two different Operating Systems, named guest OS and host OS, so that you can set up a different software environment on the same machine.
