Each notebook or desktop or server is run by an Operating System - the low-level software which interacts with the computer hardware, including peripherals such as a mouse or a printer, along with 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 web server, every single application runs within the parameters established by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard website scripts and server-side software like a media server. If a virtual server is set up on a physical one, there can be two independent Operating Systems, referred to as guest OS and host OS, so that you can set up a different software environment on the same machine.