An OperatingSystem (often Linux or Solaris) set up in a VirtualMachine or something akin to it, allowing to run a multitude of VirtualPrivateServers on a single computer. This is a very useful technique for ISPs and other 3rd party server providers, which can consolidate many servers onto BigIron, making their hardware as well as their customers' servers easier to manage.
RimuHosting offers Linux VPSs.
Various software projects exist that implement this strategy:
Some articles comparing various virtual server projects
Overview is a blog entry with a short description and links to Colinux, QEMU, XEN & UMLHow Xen is Different is a page on Xen's Unofficial Wiki. Probably slightly bias towards Xen. Interesting because it lists pros and cons of xen, UML, virtuosso, vmware and vserver. Bias means it is from a xen fan's POV.Xen Perfomance is again found on Xen's site. Xen's performance is compared with native linux,UML, and VMWare.Why VServer instead of UML or Xen: Goes over briefly why you would choose VServer over UML or Xen.