What is the difference between VMware ESX and VMware ESXi?

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What is the difference between VMware ESX and VMware ESXi?

http://www.vmware.com/products/vsphere/esxi-and-esx/faqs.html

VMware ESX and ESXi 4.1 Comparison

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1023990

 VMware ESX Server Versions

http://en.wikipedia.org/wiki/ESX_Server#Versions