Historically one server contained one operating system and supported one application. When a company needed another application, another server was added. Common applications include: file sharing, database, accounting, email, web server, etc.

Servers were sized for the highest utilization expected and to support the growth expected over the projected life of the server. This resulted in many servers running at approximately 15% of capacity.

Enter the Virtual Machines. In the appropriate situation, virtual machines allow much higher utilization of resources (servers, cooling, electricity, real estate, staff time, etc.). A typical server may be able to support 4 or more virtual machines, raising utilization from 15% to 60% or higher. Servers of sufficient size can support 100 or more virtual machines.

If you spotted a series of locomotives one following the other each pulling just one rail car you can imagine a typical physical server supporting one application. Many businesses in effect have a series of mini-trains. The virtual machine is is roughly equivalent to one locomotive pulling a series of rail cars behind it. Trains are exceptionally energy efficient due to this design. Visualization is a way for businesses to likewise dramatically increase their efficiency.

In addition, there are other benefits. Virtual machines can more easily be moved to a different physical machine. The virtual machine does not “care” that the new physical computer is a different brand or has different components; similar hardware is not required. Updates and patches to the application and the operating system can be easily verified on a test virtual machine before applying to the production virtual machines.

VMware, Microsoft, and Citrix are sources of virtualization software.

http://www.foresightcomputersolutions.com/system_upgrades/virtual-machines/vmware/

http://www.microsoft.com/windowsserver2008/en/us/hyperv-main.aspx

http://www.citrix.com/English/ps2/products/product.asp?contentID=1857200