Have a look at these
and this:
Cloud computing is not so much a technology but a system built around four basic concepts: Abstraction - the user does not need to know the underlying hardware that is running the system Elasticity - resources can be added and removed from the system easily Democratisation - users can allocate more or less resources without needing administrator assistance Utility pricing - users can get resources without upfront capital outlay, simply paying a monthly fee for resources used, like you do with utility bills (electricity, etc)
from here http://serverfault.com/questions/94057/how-we-build-private-cloud-computing-in-my-organization
