Virtualization is the act of abstracting or removing a layer of complexity from a given object, in relation to a specific resource.
There are many forms of virtualization, but recently the term has emerged to describe concepts in Information Technology and Computer Science. 
- Virtual PC (simulate XP, Vista, Server, etc in Windows 7): http://www.microsoft.com/windows/virtual-pc/
- Virtual PC 2007 (for Windows XP and Vista): http://www.microsoft.com/windows/virtual-pc/support/virtual-pc-2007.aspx
- Internet Explorer Application Compatibility VPC Image (run various IE versions): http://www.microsoft.com/downloads/en/details.aspx?FamilyID=21eabb90-958f-4b64-b5f1-73d0a413c8ef
- How to determine whether a computer is running a 32-bit version or 64-bit version of the Windows operating system: http://support.microsoft.com/kb/827218/en-us