According to a paper published by IEEE Internet Computing in 2008 "Cloud Computing is a paradigm in which information is permanently stored in servers on the Internet and cached temporarily on clients that include computers, laptops, handhelds, sensors, etc.
In Computer science, Cloud computing is a marketing term. In the case of electricity, users can simply use it. They do not need to worry where the electricity is from, how it is generated, or transported. At the end of the month, they will get a bill for the amount of electricity they consumed.
The idea behind cloud computing is similar: The user can simply use storage, computing power, or specially crafted development environments, without having to worry how these work internally. Cloud computing is a systems architecture model for Internet-based computing. It is the development and use of computer technology on the Internet. The cloud is a metaphor for the Internet based on how the internet is described in computer network diagrams; which means it is an abstraction hiding the complex infrastructure of the internet. It is a style of computing in which IT-related capabilities are provided “as a service”, allowing users to access technology-enabled services from the Internet ("in the cloud")without knowledge of, or control over the technologies behind these servers.
Cloud computing is a general concept that utilizes software as a service (SaaS), such as Web 2.0 and other technology trends, all of which depend on the Internet for satisfying users' needs. For example, Google Apps provides common business applications online that are accessed from a web browser, while the software and data are stored on the Internet servers.
Cloud computing is being driven by providers including Google, Amazon.com, and Yahoo! as well as traditional vendors including IBM, Intel, Microsoft and SAP.
Capital expenditure minimized, therefore low barrier to entry as infrastructure is owned by the provider and does not need to be purchased for one-time or infrequent intensive computing tasks. Services are typically being available to or specifically targeting retail consumers and small businesses.
Device and location independence which enables users to access systems regardless of location or what device they are using (for example PC, mobile,... etc).
Multitenancy enabling sharing of resources (and costs) among a large pool of users, allowing for:
Centralization of infrastructure in areas with lower costs (eg real estate, electricity)
Peak-load capacity increases (users need not engineer for highest possible load levels)
Utilization and efficiency improvements for systems that are often only 10-20% utilised.
Performance is monitored and consistent but can be affected by insufficient bandwidth or high network load.
Reliability by way of multiple redundant sites, which makes it suitable for business continuity and disaster recovery, however IT and business managers are able to do little when an outage hits them. Historical data on cloud outages is tracked in the Cloud Computing Incidents Database.
Scalability which meets changing user demands quickly, without having to engineer for peak loads. Massive scalability and large user bases are common but not an absolute requirement.
Security which typically improves due to centralization of data, increased security-focused resources, etc. but which raises concerns about loss of control over certain sensitive data. Accesses are typically logged but accessing the audit logs themselves can be difficult or impossible.
Sustainability through improved resource utilisation, more efficient systems and carbon neutrality.