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Do you have any specific usage in mind? Virtualization itself is a broad term. Virtualization applied to applications, computers, storage and many other concepts inclding iPods. Common application of virtualization.
#1 Disaster recovery
#2 Education, Testing and training
#3 Server consolidation (many physical servers into fewer server
#4 Hardware Level virtualization using Intel and AMD boxes / T1 hypervisor
#5 Hosting multiple web sites (VPS) / Root server etc
Personally, I love to use Vmware because of its support, ease of use and disaster recovery features. My personal home box runs many instance of Linux distro and Windows using XEN.
Hope this helps!
PS: Citrix Virtualization and Xen are same things (Citrix brought Xensource company). Also, Oracle VM = Redhat VM = XEN VM are same thing sponsored and supported by same different commercial entities