Virtualisation is the creation of workstation or server environments that run on another host platform and are not directly linked to the hardware on which they operate.
A virtual machine is a single file which means it can be saved, copied, backed up and moved to a different location very rapidly. Complete systems (applications, OS, BIOS and even virtual hardware) can be moved from one server to another in seconds.
A virtualised desktop is indistinguishable from a regular desktop and many such desktops can be hosted on a single server meaning reduced hardware, software, roll-out and maintenance costs as well as a reduced carbon footprint (there are less physical machines required).
Many virtualised servers can also run on a single host meaning it is possible to consolidate many servers (e.g. email servers, web servers) that are needlessly running on individual machines onto a single piece of hardware.
The main benefits of virtualisation are: