The National Institute of Standards and Technology (NIST) defines cloud computing as a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources, such as networks, servers, storage, applications, and services, that can be rapidly provisioned and released with minimal management effort or service provider interaction.1
NIST defines three primary service models:
Software as a service (SaaS). Applications that are delivered as a service to end users over the internet. Oracle CRM On Demand is an example of SaaS. SaaS is also known as renting hosted applications, often delivered by an application service provider.
Platform as a service (PaaS). An application development and deployment platform delivered as a service to developers, enabling them to quickly build and deploy SaaS applications. These platforms are often built on a grid computing architecture and include database and middleware. Cloud service providers such as RackSpace use these assets to develop and offer their own types of cloud services.
Infrastructure as a service (IaaS). Hardware such as compute servers, storage, and networking gear delivered as a service. This often includes associated software such as operating systems, virtualization, and clustering. Good examples of IaaS are Amazon Web Services' Amazon Elastic Compute Cloud for compute servers and Amazon Simple Storage Service for storage.
1The NIST Definition of Cloud Computing, version 15, October 7, 2009