Every single PC or server is run by an Operating System - the low-level software that interacts with the hardware equipment, including peripheral devices such as a mouse and a printer, as well as 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 OS. On a web server, any 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 Internet site scripts and server-side software like a media server. If a virtual server is created on a physical one, there can be two different Operating Systems, called guest OS and host OS, so you can set up a different software environment on the same machine.