Every personal computer or web server is run by an Operating System - the low-level software which interacts with the hardware equipment, including peripherals such as a mouse or a printer, along with any applications that are present on the 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, any app runs within the parameters established 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 created on a physical one, there can be two independent Operating Systems, called guest OS and host OS, which means that you will be able to set up a different software environment on a single machine.