Traditionally, servers were built and operated on individual machines.
This also meant corresponding financial not inconsiderable expense.
Today, entire server farms are grouped together in so-called virtualization clusters.

That is, a Windows or Linux server is no longer physically available, but built as a virtual machine on a virtualization host.
There are several types of virtualization systems, but the leading virtualization software is VMWARE ESXi.

ESX servers can be clustered together with other ESX servers.

This allows you to create a HA solution.

Servers hosted on VSSE are based on this system.