NIST Definition

Cloud Computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources that can be rapidly provisioned and released with minimal management effort or service provider interaction. This cloud model promotes availability and is composed of the following:

Essential Characteristics:

Service Models:

Cloud Software as a Service (SaaS):
Cloud Platform as a Service (PaaS): provide computational resources via a platform upon which applications and services can be developed and hosted. Examples: Force.com, Google App Engine, Windows Azure.
Cloud Infrastructure as a Service (IaaS): provide resources as services to the user. Examples: Amazon EC2/S3, SQL Azure, Zimory, Elastichosts.

Deployment Models:

Berkeley Definition

Cloud computing refers to both the applications delivered as services over the internet and the hardware and systems software in the datacenters that provide those services.
The services themselves are called Software as a Service (SaaS).
The datacenter hardware and software is called Cloud.
Types of clouds,
- Public Cloud: A cloud that is made available to the public in a pay-as-you-go manner.
  - The service being sold here is called Utility Computing.
- Private Cloud: The internal datacenters of a business or other organization which are not made available to the public.
The terms Hardware as a Service (HaaS), Infrastructure as a Service (IaaS) and Platform as a Service (PaaS) are not precisely defined.