Virtualization
The virtualization of computer systems is to operate on the same machine (physics) multiple operating systems simultaneously. Specifically, an operating system, so-called host is installed on the machine "physical" and emulates one or more machines "virtual" (with processor, memory, hard disk, network card, BIOS, ...). Each virtual machine can host an operating system called "virtualized" (or "guest"). The host system provides the partitioning between virtualized systems and the sharing of physical resources: it is a "supervisor".
Note: the host and guest operating systems can all be different: there may be a supervisor Linux, a Windows 2003 server guest system and another system called Mac OS X.
Virtualized systems are manipulated at will by the supervisor: start, stop, freeze, saving context ... In addition, as the hard drive of a virtualized system is usually emulated by a file, it is easy to duplicate a virtualized system or do migrate from one host to another. Virtualization makes it possible to make optimal use of resources of a machine can add virtual machines if the physical machine is underutilized (saving money by pooling of resources) or delete if it is saturated (management of scalability). Finally, virtualization allows a distribution of services on multiple virtual machines and thus better security: if one system is compromised, the others can continue to function normally.
