Success is walking from failure to failure with no loss of enthusiasm - Winston Churchill
VMWare offerings here can be split into four categories:-
Layer 1: ESXi hypervisor – bare metal hypervisor, like Citrix XenServer. This is VMware’s loss leader; VMware gives it out for free, while making most of its revenue on augmenting ESXi functionality with vSphere bundles.
Layer 2: vSphere – vSphere is essentially a commercial offering of ESXi which includes the hypervisor and a set of enterprise stuff, such as tools for live migration, advanced security, network and storage I/O control etc.
Layer 3: vCenter – To run vCenter the user should also purchase some vSphere bundle. In addition to vCenter Server, VMware offers vCenter plug-in modules such as Recovery Manager, Lab Manager (to manage DevOps environments), Chargeback etc.