Every desktop or notebook or server is run by an Operating System - the low-level program that interacts with the hardware, including peripherals such as a mouse or a printer, along with any apps that are present 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, each and every 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 such as a media server. In case a virtual server is created on a physical one, there can be two separate Operating Systems, called guest OS and host OS, which means that you will be able to set up a different software environment on the same machine.