Cloud computing is a general term for anything that involves delivering hosting services over the Internet. A cloud service has three distinct characteristics that differentiate it from traditional hosting. It is sold on demand, typically by the minute or by the hour, it is elastic – a user can have as much or as little of a service as they want at any given time; and the service is fully managed by the provider.
Here is a graphical look at the cloud computing landscape. From private and enterprise cloud providers (such as IBM, HP/3Par, and VMware), to public cloud services (like Google and Microsoft Azure), to the Hybrid cloud (Verizon and the NRE Alliance of newScale, rPath and Eucalyptus as examples).