But wait! What's a cloud?
Fortunately, the US National Institute of Standards and Technology [NIST] felt compelled to write a nice concise definition of cloud computing, the gist of which, as reported on Fierce Government IT, is given below:
On demand self-service that allows consumers to unilaterally provision computing capabilities without human interaction with the service provider,
Broad network access, meaning that capabilities are available over a network and can be accessed by heterogeneous platforms, i.e., not just a dedicated thin client.
Resource pooling such that different physical and virtual resources get dynamically assigned and reassigned according to consumer demand in a multi-tenant model.
Rapid elasticity so that to the consumer, available capabilities often appear to be unlimited and can be purchased in any quantity at any time.
Measured service allowing usage it be monitored, controlled and reported and automatically controlled and optimized
In addition, NIST says cloud service models exist in three varieties:
Cloud software as a service, in which applications run on a cloud but the user doesn't provision or modify the cloud service, or even application capabilities, apart from limited user-specific configuration settings.
Cloud platform as a service, in which users can utilize cloud-provided programming tools to deploy applications without controlling most of the underlying infrastructure, with the possible exception of the application hosting environment configuration.
Cloud infrastructure as a service might be termed the whole nine yards of cloud computing, except that NIST would never be so colloquial. Under it, the consumer has control over the operating systems, storage, deployed applications, and possibly limited control of select networking components (e.g., host firewalls) of the cloud environment available to the user via the network
Finally, NIST also says there are four deployment models:
A private cloud in which the cloud infrastructure is utilized by just one organization, though not necessarily operated by that one organization.
A community cloud whereby several organizations with common concerns share a cloud.
The public cloud provided by the private sector for all comers (which, although NIST doesn't say this, the government on occasion seems to believe consists entirely of Amazon Web Services).
A hybrid cloud in which two or more cloud types are discrete but networked together such that a burst of activity beyond the capabilities of one cloud is shifted for processing to another.
Image: NOAA.gov
