Cloud Computing Defined

Jeff Kaplan takes a considered view of cloud computing and with the simplicity bourne from great experience defines Cloud Computing as "a set of web-based tools and services which permit users to acquire computing resources and development capabilities to build or support applications, or perform specific IT functions on a pay-as-you-go basis

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While that's a decent IT-centric view of the world, infact I would modify it slightly by stating that Cloud Computing allow users to perform business functions on pay-as-you-go basis. Since the role of any IT function is to support or enhance business processes, this higher-level of extrapolation serves to focus the mind on cloud-computing being a solution for business and not necessarily only for IT.