Cloud computing is, in general, using remote servers whose specifics are unknown to you. Typically, this is using server resources in a large mainframe cluster. You don't know where the server is, what its physical properties are, or nearly anything about it. What you do know, is its general power. It's using remote resources in an expandable and configurable way."
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http://ruby.about.com/od/reviewsevents/a/cloudcomputing.htm