National Institute of Standards and Technology (NIST) provides a concise and specific definition:
Cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction.
Cloud computing provides computation, software, data access, and storage services that do not require end-user knowledge of the physical location and configuration of the system that delivers the services. Parallels to this concept can be drawn with the electricity grid, where end-users consume power without needing to understand the component devices or infrastructure required to provide the service.
Cloud computing describes a new supplement, consumption, and delivery model for IT services based on Internet protocols, and it typically involves provisioning of dynamically scalable and often virtualized resources It is a byproduct and consequence of the ease-of-access to remote computing sites provided by the Internet. This may take the form of web-based tools or applications that users can access and use through a web browser as if they were programs installed locally on their own computers.
Cloud computing providers deliver applications via the internet, which are accessed from a Web browser, while the business software and data are stored on servers at a remote location. In some cases, legacy applications (line of business applications which until now have been prevalent in thick client Windows computing) are delivered via a screen sharing technology such as Citrix XenApp, while the compute resources are consolidated at a remote data center location; in other cases entire business applications have been coded using web based technologies such as AJAX.
Most cloud computing infrastructures consist of services delivered through shared data-centers. The Cloud may appear as a single point of access for consumers' computing needs, notable examples include the iTunes Store, and the iPhone App Store. Commercial offerings may be required to meet service level agreements (SLAs), but specific terms are less often negotiated by smaller companies.
