What's the difference between VMware vCenter and vSphere?

What's the difference between VMware vCenter and VMware vSphere? To answer this question, let's put it in a broader context. As a VMware Certified Instructor, I teach newbies about VMware's products and they're always asking me what the difference is between VMware ESXi and vSphere and then, of course, vCenter.

ESXi is the actual hypervisor that runs on bare-metal hardware. It's the OS organizations install -- just like Windows or Linux -- to control their hardware and applications. But in this case, it isn't a generic OS but a specific one that runs the virtualization layer known as the hypervisor. ESXi also controls the hardware and runs VMs. There's a free edition of ESXi that IT administrators can download and use.

But with this free edition, admins can only run VMs, nothing more. There's no management or fancy features such as vMotion or high availability. For those and other features, an organization must purchase a vSphere license. Adding a vSphere license to the host makes the ESXi server into a vSphere host. The features included depend on the type of license the organization purchases: Standard, Enterprise Plus or Platinum. 

For smaller environments that have up to three servers, the Essentials Kit and Essentials Plus kit come with a license for vCenter and a maximum of three vSphere hosts with two CPUs each.

But just a license isn't enough. To use vSphere features, including the aforementioned ones, as well as templates and alarms, an organization must also run a vCenter server. This software centrally manages all vSphere hosts. But vCenter requires its own license. For smaller environments that have up to three servers, the Essentials Kit and Essentials Plus kit come with a license for vCenter and a maximum of three vSphere hosts with two CPUs each. 

The free ESXi hypervisor can't be managed with vCenter. So an organization must purchase a combination of vSphere licenses and a vCenter license to work with VMs in a vSphere environment.

vCenter should be the single point of management for all vSphere hosts and VMs. For that reason, it can also be connected to Active Directory or other Lightweight Directory Access Protocol directories to enable admins to login with their named account and perform allowed tasks.