Virtualization

Virtualization is Article description::the concept and technique that permits running software in an environment separate from a computer operating system.

The operating system actually running on the hardware is referred to as the host. On this host resides a hypervisor (aka virtual machine manager), which runs virtual machines containing guest software.

Containers
Containers provide isolated user space instances.

Guest facilities
Most virtualization systems provide software to be installed inside the guest operating systems, to provide extra functionality.

These packages are for Gentoo guests running inside virtual machines. See respective hypervisor documentation for tools helping to run other operating systems inside virtual machines.