"The concept of the 'cloud' comes from network architecture drawings of modern telephone systems and later the Internet. The early cloud service providers included Amazon Web Services, and Google, which uses it for Gmail and several other applications and services.
"Today, cloud computing typically includes three main offerings (in order):
SaaS (Software-as-a-Service). Delivers applications and end-user tools, such as ecommerce applications, and other brands like Google Apps and Salesforce.com.
PaaS (Platform-as-a-Service). Provides pre-built technology frameworks and development tools, such as Engine Yard AppCloud or xCloud, Microsoft Azure, and Google App Engine.
IaaS (Infrastructure-as-a-Service). Enables computing, storage and network resources to be provisioned without owning physical devices. Example IaaS providers are Amazon Web Services and Terremark."