The Ubuntu Enterprise Cloud (UEC) allows you to build your own private cloud on existing hardware platforms that already run (or can run) Ubuntu Server, which is pretty much most of the Intel-based servers you have on hand. UEC is really just an implantation of the Eucalyptus cloud computing architecture, which is interface-level compatible with Amazon.com's cloud. This means that most who understand and deal with AWS will find UEC to be an on-premise extension of that technology, generally speaking.
The UEC architecture uses front-end computing that functions as the "controller," and one more "node" system using either KVM or Xen virtualization technology for running system images. Note that you cannot use just any OS image; it has to be prepared for use within UEC. A few basic images are provided by Canonical, the developer of UEC.
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The larger selling point here is the on-premise compatibility with Amazon Web Services. Those who use Amazon.com's EC2 -- there are legions of such users right now -- will be familiar with Amazon.com's S3, which allows you to persist data for use in the cloud. Eucalyptus offers a similar technology, called Walrus, which is interface-compatible with S3. Thus, if you're already an S3 shop, moving to this technology should not be much of a stretch.