Oracle VM VirtualBox is an x86 virtualization software package maintained by Oracle Corporation as part of its family of virtualization products. VirtualBox is installed on an existing host operating system.
The physical computer hardware and operating system you have before you install VirtualBox is the "host computer" and "host operating system".
Inside the VirtualBox application, you can create many virtual machines sometimes called "guest computers" which act much like physical computer hardware, except you can't touch it because it's all simulated inside your real physical computer hardware.
Within the VirtualBox application, additional guest operating systems, each known as a Guest OS, can be loaded and run, each with its own virtual environment.