Platform as a Service (PaaS)
The capability provided to the customer is to deploy onto the cloud infrastructure customer-created or acquired applications created using programming languages and tools supported by the provider. The customer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, or storage, but has control over the deployed applications and possibly application hosting environment configurations.
Google App Engine offers users the ability to build and host web applications on Google's infrastructure. The integrated application development platform (java and python runtime environment) help speed application creation and deployment. App Engine eliminates many of the system administration and development challenges involved in building applications that can scale to million of users.
Microsoft Azure is a platform for developers to create cloud applications or enhance existing applications with cloud-based capabilities. The main components are Windows Azure (Windows-based environment for running applications and storing data on servers in Microsoft data centers), SQL Azure (data services in the cloud based on SQL Server) and Windows Azure platform AppFabric (cloud services for connecting applications running in the cloud or on premises)
Force.com is a platform developers can use to build, deploy, and manage applications in the cloud that leverages the Salesforce.com infrastructure. Force.com includes workflow, database, user interface, and other tools designed to build business applications, Web sites, and mobile applications.
