due to Communications of the ACM: http://cacm.acm.org/magazines/2010/4/81493-a-view-of-cloud-computing/fulltext
This is a good overview that makes several key points that help with the definition of cloud computing, and related nomenclature and classification. I believe that the hype around cloud is so high that many vendors and solution providers bend over backwards to define cloud computing so that whatever they are pitching is a key cloud component you can't live without. Articles like this help provide a framework to sort out the clutter.
Their list of 10 obstacles and opportunities is particularly well-done and insightful.