Running numerous web sites from different customers on a single server has been well understood for the past 15 years or so, and it has been the mainstay of hosting providers worldwide. This model of web hosting allowed small businesses and individuals to create cost-effective online presences. In the last 10 years, however, emulating hardware to allow entire computer operating systems (OSs) to run has enabled hosting scenarios that, without virtualization, would previously have been too costly. Virtual machines (VMs) emulate the hardware required to run computer OSs. The VMs discussed in this book are all system virtual machines, not process virtual machines; process VMs are part of a detailed software development conversation. Only OSs supported on virtual hardware architecture can be run in a VM. Microsoft uses only x64 hardware for their hosts; hence any x86/x64 workload can run on the VMs running on these hosts. These VMs are also frequently referred to as . A VM is basically a software version of a computer’s hardware onto which you can install an OS; the host slices its resources across all the VM guests it has running. For more information about VMs and how their abstraction works, see http://en.wikipedia.org/wiki/Virtual_machine . You can also download a copy of Microsoft’s “Virtualization for Windows: white paper at http://download.microsoft.com/download/8/6/2/862032E0-9D03-4AB5-B033-E6022A6186B1/VirtualizationforWindows_a_Technology_overview.pdf .