What is Virtualization?
Virtualization in the technology world is the utilization of computer/server hardware to provision multiple operating systems (or containers) that run simultaneously on a single system.
You may have done this on your own using Virtualbox, VMWare or similar software.
As an example, you may have Virtualbox running with numerous virtualized operating systems. You could potentially have different versions of Windows Servers, Linux Servers, and other operating systems for testing, development or learning.
This can be done from one physical machine.
This can be accomplished on servers very easily (and is frequently used by businesses).
Windows Servers include Hyper-V by default (needs to be installed). VMWare (VSphere) is one of the most popular alternatives to Hyper-V.