difference

VMWare vSphere – Demystifying what is FREE

FREE: VMware vSphere Hypervisor   (Also known as ESXi)

ADV: Free  VMware vSphere Client to administer the server

DISAD: Cannot be managed with vCenter or other management solutions

COMMERCIAL: VMware vSphere

ADV: advanced functionally including centralized management, live migration of virtual machines, automatic load balancing, business continuity, power management, and backup and restore capabilities for virtual machines.

Hence, The extra word “Hypervisor” in the product name is the mystery & decides which one is free & which one must be purchased.

Free can be upgraded by putting a license.

Source:

http://www.vmware.com/products/vsphere-hypervisor/faq.html