This portal talks about virtualization. Technical language could be used.
In computing, platform virtualization is a term that refers to the abstraction of computer resources. Virtualization hides the physical characteristics of computing resources from their users (more on Wikipedia)
Note that virtualization is different than emulation, which duplicates (provides an emulation of) the functions of one system using a different system, so that the second system behaves like (and appears to be) the first system (more on Wikipedia)
Using Debian to host Virtual Computers
Open Source solutions
Qemu - Mostly used on Desktops/Laptops.
KVM - Used on servers as well as Desktops/Laptops.
VirtualBox - Mostly used on Desktops/Laptops. Similar to VMWare Workstation. Can be used in headless mode on servers.
Xen - Provides para-virtualization and full-virtualization. Mostly used on servers.
LXC - Linux Containers. Creates multiple isolated containers. Mainline since 2.6.26.
LinuxVserver - Mostly used on servers. Creates multiple isolated containers. DEPRECATED.
OpenVz - Mostly used on servers. Creates multiple isolated containers. DEPRECATED.
UserModeLinux Not really a virtualization. Doesn't need superuser privileges.
libvirt - Manage different virtualization solutions such as KVM and Xen through a common (programming and user) Interface.
Ganeti - Cluster virtualization manager, for KVM or Xen.
VMware - Virtualization tool.
VirtualBox - There is an Open-Source-Edition (OSE) and a commercial one with enhanced functionality.
Running Debian inside a Virtual computer
Debian inside Microsoft Virtual PC 2007.
Debian inside Microsoft Hyper-V.
Information on how to configure Debian to work with various virtualization methods simultaneously is available on the VirtualNetworking page.
virsh - A command line tool based on libvirt-bin offering complete virtual infrastructure management.
xmount - tool to crossmount between multiple input and output harddisk images