Every PC or web server is run by an Operating System - the low-level software which communicates with the hardware, including peripheral devices such as a mouse or 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 server, every single app runs within the parameters set by the OS as well - priority, physical memory, processing time, etcetera. This is valid for both standard Internet site scripts and server-side software like a media server. When a virtual server is generated on a physical one, there can be two different Operating Systems, referred to as guest OS and host OS, so you'll be able to set up a different software environment on a single machine.