Hopefully this set of statements helps you:
- ESXi is the software you run on each single physical system to host VMs.
- vCenter Server is how you manage multiple ESXi hosts.
- vSphere is the collective name given to a setup where you have vCenter Server and multiple ESXi hosts.
- You must use vCenter Server if you want to use vMotion, High Availability (HA), Distributed Resource Scheduler (DRS), and many many more vSphere features.
- HA and DRS are the features where you create host clusters.
- ESXi hosts are licensed with vSphere licenses.
- You can also use ESXi on a standalone system (not managed by vCenter Server) with a free license - that's when ESXi is called vSphere Hypervisor.
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