There are at least three forms of "cloud computing," each with different benefits and risks. First, there is "infrastructure as a service" (bare-metal virtual servers available on demand from the likes of Amazon's Elastic Compute Cloud. Second, there are Web services providers, or "platform as a service," which are APIs or development platforms that let customers create and run apps in the cloud. And third, there is software as a service, applications such as Salesforce.com's CRM software that users access over the internet with little or no code running on their own machines.
Source: CIO