Linux had always lacked a Open Source virtualisation technology in the same league as Solaris containers or commercial product like Vmware. That was until Xen came into the picture. Xen is an opensource virtual machine monitor for x86 that supports execution of multiple guest operating systems. Xen is released under the GPL and can easily be used to run virtually, OSes as diverse as different Linux distributions, BSD's and even windowsXP (though windows port is not available because of licencing restrictions). Virtualisation technologies are nothing new, what with Vmware, Usermode Linux, Win4Lin and others available. But Xen is relevant here because of the support for it from Redhat, GPL-ed licence and also its active development.
http://linuxhelp.blogspot.com/2006/10/xen-gpled-virtualisation-technology.html