From "The various level of cloud computing" by Ross Cooney
Applications in the cloud: Software as a Service (SaaS). Examples include gmail, yahoo mail, Hotmail, the various search engines, wikipedia, encyclopedia britannica, etc.
Platforms in the cloud: Developers write their application to a open specification and then upload their code into the cloud where the app is run on the cloud infrastructure with automatic scale-up as application usage grows. Examples include Mosso, Google App Engine, and Force.com
Infrastructure in the cloud: Developers and system administrators obtain general compute, storage, queueing, and other resources and run their applications with the fewest limitations. This is the most powerful type of cloud in that virtually any application and any configuration that is fit for the internet can be mapped to this type of service. an example of this is Amazon Web Services