Cloud computing is a general term for anything that involves delivering hosted services over the Internet. These services are broadly divided into three categories: Infrastructure-as-a-Service (IaaS), Platform-as-a-Service (PaaS) and Software-as-a-Service (SaaS). The name cloud computing was inspired by the cloud symbol that's often used to represent the Internet in flowcharts and diagrams.

A cloud service has three distinct characteristics that differentiate it from traditional hosting. It is sold on demand, typically by the minute or the hour; it is elastic -- which means that a user can have as much or as little of a service as they want at any given time; and the service is fully managed by the provider (the consumer needs nothing but a personal computer and Internet access). Significant innovations in virtualization and distributed computing, as well as improved access to high-speed Internet and a weak economy, have accelerated interest in cloud computing.

A cloud can be private or public. A public cloud sells services to anyone on the Internet. (Currently, Amazon Web Services is the largest public cloud provider.) A private cloud is a proprietary network or a data center that supplies hosted services to a limited number of people. When a service provider uses public cloud resources to create their private cloud, the result is called a virtual private cloud. Private or public, the goal of cloud computing is to provide easy, scalable access to computing resources and IT services.

Infrastructure-as-a-Service provides the customer with virtual server instances and storage, as well as application program interfaces (APIs) that allow the customer to start, stop, access and configure their virtual servers and storage. This model allows a company to pay for only as much capacity as is needed, and bring more online as soon as required. Because this pay-for-what-you-use model resembles the way electricity, fuel and water are consumed, it's sometimes referred to as utility computing.

Platform-as-a-service in the cloud is defined as a set of software development tools hosted on the provider's infrastructure. Developers create applications on the provider's platform over the Internet. PaaS providers may use APIs, website portals or gateway software installed on the customer's computer. Force.com, (an outgrowth of Salesforce.com) and GoogleApps are examples of PaaS. Developers need to know that currently, there are not standards for interoperability or data portability in the cloud. Some providers will not allow software created by their customers to be moved off the provider's platform.

In the software-as-a-service cloud model, the vendor supplies the hardware infrastructure, the software product and interacts with the user through a front-end portal. SaaS is a very broad market. Services can be anything from Web-based email to inventory control and database processing. Because the service provider hosts both the application and the data, the end user is free to use the service from anywhere.

• Storage as a Service (STaaS): storage will be offered in cloud computing. Any user doesn’t need to store the date on his own system but it  can be stored somewhere else if the user has not sufficient storage capacities

Meghraj (GI Cloud Initiative)

• It is the name given to the initiative of Government of India for its new program which is going to take advantage of the Cloud Computing. It will enable the government to leverage cloud computing for effective delivery of e-services
• The GI Cloud will provide services to government departments, citizens and businesses through internet as well as mobile connectivity.
• In addition to accelerating the delivery of e-services to citizens and businesses , the government’s cloud-based service delivery platform will also support a number of other objectives including increased standardization, interoperability and integration

Advantages of GI Cloud

• Optimum utilization of existing infrastructure
• Rapid deployment and reusability: Any software made available by any government of department in India can be made available to other departments as well without additional costs.
• Manageability and maintainability: It provides single point for maintaining Information & Communication Technology (ICT) infrastructure in India.
• Scalability: According to the demands from the citizens of India, infrastructure of the government can be increased accordingly.
• Efficient service delivery
• Security: A security framework for the entire GI Cloud will lead to less environmental complexity and less potential vulnerability.
• Increased user mobility
• Reduced effort in managing technology
• Ease of first time IT solution deployment
• Cost reduction
• Standardization: GI Cloud shall prescribe the standards around interoperability, integration, security, data security and portability etc.

Disadvantages of Cloud computing:

• Low Bandwidth: With a low bandwidth net, the benefits of Cloud computing cannot be utilized. 
• Non-negotiable Agreements: Some cloud computing vendors have non-negotiable contracts for the companies. It can be disadvantageous for a lot of businesses.
• Incompatibility: Sometimes, there are problems of software incompatibility. As some applications, tools, and software connect particularly to a personal computer.
• Technical Issues-prone to outages and other technical issues. Even the best cloud service providers run into this kind of trouble, in spite of keeping up high standards of maintenance. Besides, you will need a very good Internet connection to be logged onto the server at all times.
• Security issues-Before adopting this technology, you should know that you will be surrendering all your company’s sensitive information to a third-party cloud service provider. This could potentially put your company to great risk. Hence, you need to make absolutely sure that you choose the most reliable service provider, who will keep your information totally secure.
• Prone to Attack-Storing information in the cloud could make company vulnerable to external hack attacks and threats. 

Grid Computing

Grid systems are designed for collaborative sharing of resources. It can also be thought of as distributed and large-scale cluster computing. A Grid is basically the one that uses the processing capabilities of different computing units for processing a single task. The task is broken into multiple sub-tasks, each machine  on a grid is assigned a task. As when the sub-tasks are completed they are sent back to the primary machine which takes care of the all the tasks. They are combined or clubbed together as an output.