Hi,
Regards
VSphere - is a hypervisor which allows users to install multiple virtual machine instances running different (or same) operating systems.
vCenter - is a product that can be used to manage and administer multiple vsphere hosts
vCloud - take a look at http://en.wikipedia.org/wiki/VCloud for a better definition than i could give
vCloud Director - http://www.yellow-bricks.com/2010/08/31/vmware-vcloud-director-vcd/
Many tx!
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