Cloud Computing is the use of hardware and software to deliver a service over a network (typically the Internet). With cloud computing, users can access files and use applications from any device that can access the Internet.

There are 3 services that are fundamentally used in cloud computing:

• Infrastructure as a service (IAAS)
• Platform as a service (PAAS)
• Software as a service (SAAS)

If you have a web based mail account hosted by Gmail, Yahoo, Hotmail, Rediffmail etc, then you have already experienced cloud computing. Instead of storing the mails on your computer It store them some where else, you use a browser to connect to the Internet and access the mails. The software and storage for your account doesn’t exist in your computer. This is one of the cloud computing service. You as an user simply access the services in cloud with your computer browser. But, in the back end there are various computers, servers and data storing systems that create the whole cloud computing system.
Infrastructure as a service (IAAS) :
In Infrastructure as a service (IAAS) the provider manages and delivers the underlying infrastructure, including storage, network and computing resources which can be used by software developers for building business solutions. Cloud users can install their operating system and their applications on the cloud infrastructure. Example of the companies that provide IAAS are Amazon EC2, Google compute engine, HP cloud etc.
Platform as a service (PAAS):
Platform as a service (PaaS) is a category of cloud computing services that provides a computing platform on top of which you can build a application as desired. The platform can include operating system, programming language, frameworks, libraries, services database and and other services that are required to host your developed application. Service provider could offer a developer tool which can be used to build services or database services or data storage services on the provided platform. Example of PAAS service providers are Force.com, Heroku, Azure cloud services, Google app engine.
Software as a service (SAAS):
Software as a service (SAAS) is a way of delivering applications over the Internet—as a service. Instead of installing and maintaining software, you simply access it via the Internet, freeing yourself from complex software and hardware management. An example of application service is CRM or ERP. SAAS delivers single application through the browser to number of customers using a multitenant architecture environment. Salesforce.com is a Best known example of SAAS, that provides CRM application built on force.com. Other examples of SAAS are google apps, microsoft office 365.