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document updated 21 years ago, on Jun 3, 2005
Basically, a bunch of geeks get together, rent one or more machines, split it into many virtual machines, and give each member root (usually each having their own IP address). The benefit is that it can be colocated and be on good hardware, thereby ensuring good network speed and high uptime for CPU/disk/network.

It's often done using UML, but Xen may become a better alternative, especially with x86 hardware virtualization being supported by Xen some time this year.