The NIST draft definition lists five essential characteristics of cloud computing and goes on to explain what they are in technical terms (Mell and Grance, 2009). I have provided my own slightly simplified interpretations below, and when I refer to computing resources I mean such things as storage, processing, memory, network bandwidth, software applications and virtual machines.
On-demand self-service
Consumers can log on to a website or use web services to access additional computing resources on demand, that is, whenever they want them, without talking to a sales representative or technical support staff.
Broad network access
Because they are web-based, you can access cloud computing services from any internet-connected device. With a web browser on a PC (or even a thin client computer terminal) you can do anything, but there is also, in many cases, explicit support for popular hand-held devices such as Blackberries and iPhones.
Resource pooling
In multi-tenanted computing clouds the customers (tenants) share a pool of computing resources with other customers,
and these resources, which can be dynamically reallocated, may be hosted anywhere.
Rapid elasticity
Cloud computing enables computing resources or user accounts to be rapidly and elastically provisioned or released so that customers can scale their systems (and costs) up and down at any time according to their changing requirements.
Measured service
Cloud computing providers automatically monitor and record the resources used by customers or currently assigned to customers, which makes possible the payper use billing model that is fundamental to the cloud computing paradigm.