Cloud Computing - correctly: a Computing Cloud - is a colloquial expression used to describe a variety of different computing concepts that involve a large number of computers that are connected through a real-time communication network. Cloud Computing is a jargon term with no distinct and non-ambiguous scientific or technical term definition that is commonly accepted.
Cloud Computing in scientific use is a popular jargon term for Distributed Computing where software executes on a large number of loosely connected computers across the internet or a large enterprise network. Distributed computing allows algorithms to be executed simultaneously on a large number of inter-connected computers. In cases when the participating computers are assigned only on demand we speak of an "Elastic Cloud"; this term has been introduced and made popular by Amazon while introducing their on-demand computing platform, called the Amazon Elastic Cloud.
Cloud Computing in its wider sense is used mainly in marketing to depict any kind of software and hardware that is accessible over the internet for a third party. Here we mainly speak of classical software application and hardware hosting models that exist for long time already like application hosting (ASP-Application Service Provisioning; SaaS-Software as a Service), machine hosting (PaaS-Platform as a Service) or giving access to commodity applications like web site hosting. In this context Cloud Computing is mainly a marketing term and does not describe any new kind of technology or computing paradigm.
The expression Cloud Computing is derived from the common usage of the word "cloud" to describe an agglomeration of a large number of small particles:
• Meteorology: a weather cloud is an agglomeration of small water bubbles;
• Mathematics: a large number of points in a coordinate system in mathematics is seen as a point cloud;
• Astronomy: many stars that crowd together are seen as star clouds (also known as star mist) in the sky, e.g. the Milky Way;
• Physics: The fast movement of electrons around an atomic kernel appears like a cloud to a distant observer;
In technology the expression cloud is common for a long time e.g. in telephony diagrams where it is used to name a large number of communication equipment whose details are of no further interest. Therefore it became popular to use a cloud shaped symbol to picture an unknown aggregate of equipment.
A set of computers that form a cloud and that are publically accessible via the internet is called "Public Cloud" while a cloud confined to an enterprise only network is called "Private Cloud".
There are alternative expression that denominate distributed computing in a similar fashion.
• Crowd Computing
• Grid Computing
Cloud computing is primarily a marketing term used to promote services that are accessible via the internet and analogous installation within an enterprise network that follow the same work principle.
Making computing resources available over the internet
SaaS - Software as a Service; classical client-server architecture where software is installed on a server and accessed through a client program, usually an internet browser
PaaS - Platform as a service; renting ready-to use computers that can be accessed remotely over the internet, usually made available as virtual machines
IaaS - Infrastructure as a service; making commonly required resources like printing available over the internet