CLOUD COMPUTING
   
Cloud computing is the delivery of computing and storage capacity as a service to a community of end-recipients. The name comes from the use of a cloud-shaped symbol as an abstraction for the complex infrastructure it contains in system diagrams. Cloud computing entrusts services with a user's data, software and computation over a network.

There are three types of cloud computing

Infrastructure as a Service (IaaS),
Platform as a Service (PaaS), and
Software as a Service (SaaS).

Using Software as a Service, users also rent application software and databases. The cloud providers manage the infrastructure and platforms on which the applications run.
End users access cloud-based applications through a web browser or a light-weight desktop or mobile app while the business software and user's data are stored on servers at a remote location. Proponents claim that cloud computing allows enterprises to get their applications up and running faster, with improved manageability and less maintenance, and enables IT to more rapidly adjust resources to meet fluctuating and unpredictable business demand.

Cloud computing relies on sharing of resources to achieve coherence and economies of scale similar to a utility (like the electricity grid) over a network (typically the Internet). At the foundation of cloud computing is the broader concept of converged infrastructure and shared services.
 
Cloud computing exhibits the following key characteristics:

Agility  improves with users' ability to re-provision technological infrastructure resources.
Application programming interface  (API) accessibility to software that enables machines to interact with cloud software in the same way the user interface facilitates interaction between humans and computers. Cloud computing systems typically use REST-based APIs.
Cost  is claimed to be reduced and in a public cloud delivery model capital expenditure is converted to operational expenditure. This is purported to lower barriers to entry, as infrastructure is typically provided by a third-party and does not need to be purchased for one-time or infrequent intensive computing tasks. Pricing on a utility computing basis is fine-grained with usage-based options and fewer IT skills are required for implementation (in-house). The e-FISCAL project's state of the art repository contains several articles looking into cost aspects in more detail, most of them concluding that costs savings depend on the type of activities supported and the type of infrastructure available in-house.
Device and location independence  enable users to access systems using a web browser regardless of their location or what device they are using (e.g., PC, mobile phone). As infrastructure is off-site (typically provided by a third-party) and accessed via the Internet, users can connect from anywhere.
Virtualization  technology allows servers and storage devices to be shared and utilization be increased. Applications can be easily migrated from one physical server to another.
Multitenancy  enables sharing of resources and costs across a large pool of users thus allowing for:
        Centralization of infrastructure in locations with lower costs (such as real estate, electricity, etc.)
        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% utilized.
Reliability  is improved if multiple redundant sites are used, which makes well-designed cloud computing suitable for business continuity and disaster recovery.
Scalability and Elasticity  via dynamic ("on-demand") provisioning of resources on a fine-grained, self-service basis near real-time, without users having to engineer for peak loads.
Performance  is monitored and consistent and loosely coupled architectures are constructed using web services as the system interface.
Security  could improve due to centralization of data, increased security-focused resources, etc., but concerns can persist about loss of control over certain sensitive data, and the lack of security for stored kernels. Security is often as good as or better than other traditional systems, in part because providers are able to devote resources to solving security issues that many customers cannot afford. However, the complexity of security is greatly increased when data is distributed over a wider area or greater number of devices and in multi-tenant systems that are being shared by unrelated users. In addition, user access to security audit logs may be difficult or impossible. Private cloud installations are in part motivated by users' desire to retain control over the infrastructure and avoid losing control of information security.
Maintenance  of cloud computing applications is easier, because they do not need to be installed on each user's computer and can be accessed from different places.
On demand self service  is listed by the The National Institute of Standards and Technology (NIST) as a characteristic of cloud computing. Cloud users should be able to obtain, configure and deploy cloud services themselves using cloud service catalogues, without requiring the assistance of IT.