What is Virtualization / Virtual Machines?
A Machine Within a Machine
Virtualization is simulating the real thing. IBM invented computer virtualisation and VMWare later popularized it into the main stream. A virtual machine is a computer (machine) running inside of another computer operating system. The VM is called a guest and the host is, well, the host. VirtualBox is the most popular virtual machine software since it is free and available for all major desktop operating systems. VMWare is also available and is more stable, but has a $300+ price tag. Other options exist for Linux including Qemu and KVM.
Containerization is often confused with virtualization. Be sure to know the difference.