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Cloud computing is the next stage in the Internet's evolution, providing the means through which everything — from computing power to computing infrastructure, applications, business processes to personal collaboration — can be delivered to you as a service wherever and whenever you need.

The “cloud” in cloud computing can be defined as the set of hardware, networks, storage, services, and interfaces that combine to deliver aspects of computing as a service. Cloud services include the delivery of software, infrastructure, and storage over the Internet (either as separate components or a complete platform) based on user demand. (See Cloud Computing Models for the lowdown on the way clouds are used.

Cloud computing has four essential characteristics: elasticity and the ability to scale up and down, self-service provisioning and automatic deprovisioning, application programming interfaces (APIs), billing and metering of service usage in a pay-as-you-go model. (Cloud Computing Characteristics discusses these elements in detail.This flexibility is what is attracting individuals and businesses to move to the cloud.

·         Deployment Models:

There are Four different deployment models of cloud computing:

1)    Public Cloud:

Public or external cloud is traditional cloud computing where resources are dynamically provisioned on a fine-grained, self-service basis over the Internet or via and or from an off-site third-party provider who bills on a fine-grained basis.

2)    Community Cloud:

If several organizations have similar requirements and seek to share infrastructure to realize the benefits of cloud computing, then a community cloud can be established.  This is a more expensive option as compared to public cloud as the costs are spread over fewer users as compared to a public cloud. However, this option may offer a higher level of privacy, security and/or policy compliance.

3)    Hybrid Cloud:

Hybrid Cloud means either two separate clouds joined together (public, private, internal or external or a combination of virtualized cloud server instances used together with real physical hardware. The most correct definition of the term "Hybrid Cloud" is probably the use of physical hardware and virtualized cloud server instances together to provide a single common service. Two clouds that have been joined together are more correctly called a "combined cloud". Private Clouds: Private clouds describe offerings that deploy cloud computing on private networks.  It consists of applications or virtual machines in a company's own set of hosts. They provide the benefits of utility computing-shared hardware costs, the ability to recover from failure, and the ability to scale up or down depending upon demand.

4)      Community Cloud:

If several organizations have similar requirements and seek to share infrastructure to realize the benefits of cloud computing, then a community cloud can be established.  This is a more expensive option as compared to public cloud as the costs are spread over fewer users as compared to a public cloud. However, this option may offer a higher level of privacy, security and/or policy compliance.