SalesForce Force.com

Force.com is a cloud computing platform as a service from Salesforce.com, that developers use to build multi tenant applications hosted on their servers as a service. The platform allows external developers to create add-on applications that integrate into the main salesforce.com application and are hosted on salesforce.com's infrastructure. These applications are built using Apex (a proprietary Java-like programming language for the Force.com platform) and Visualforce (an XML-like syntax for building user interfaces in HTML, Ajax or Flex).

See the reference side-by-side comparison Platform as a service (PAAS) for cloud applications (Scalable cluster of services)