SDIA definition
We define Cloud computing as the paradigm of using commodity digital resources (see Cloud infrastructure) to perform computations - e.g. to run a software application, a function, or any form of software-driven computation.
This paradigm is different from traditional computing paradigms in which the server and equipment is dedicated or at least known to the user or the software application. In Cloud computing, the digital resources can come from anywhere (as they are commodities) - e.g. CPUs can be located in one server, and storage can be located in a different server in a different data center. In this paradigm it is neither possible nor desirable to know the physical server or location, as it would break the commodity principle of the resource.
Other definitions
Microsoft defines cloud computing as follows:
Cloud computing is the delivery of computing services – including servers, storage, databases, networking, software, analytics and intelligence – over the Internet (“the cloud”) to offer faster innovation, flexible resources and economies of scale. Typically, you only pay for cloud services you use, helping you lower your operating costs, run your infrastructure more efficiently and scale as your business needs change.
There are several different types of cloud computing to meet the needs of end users; these include: public cloud, private cloud and hybrid cloud.