Every single notebook or desktop or web server is run by an Operating System - the low-level software which communicates with the computer hardware, including peripheral devices such as a mouse or a printer, along with any applications 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 server, any application runs within the parameters established by the OS as well - priority, physical memory, processing time, etc. This is valid for both standard site scripts and server-side software like a media server. In case a virtual server is generated 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 very same machine.