People get confused with Virtualization and Cloud Technology/Computing . Its not their fault however the term are intermingled in such a way that they appear to be same. Now the question is,  If they are different then how we can distinguish ?

Answering it directly, I would rather start with a simple question. Can we create Cloud Computing without Virtualization ? And they answer is YES we can, however few of the characteristics of Cloud Computing will be performed with difficulty. So now it could be understood that Virtualization makes an ease in implementing the cloud computing.

In order to understand Virtualization in brief you may read Click here  . however in order to understand Cloud computing we should go as per NIST [National Institute of Standards and Technology , Gaithersburg, Maryland, US] definition which is symbolized in the five characteristics  :-

On-demand self-service  A consumer can unilaterally provision computing capabilities, such as server time and network storage, as needed automatically without requiring human interaction with each service provider.

Broad network access  Capabilities are available over the network and accessed through standard mechanisms that promote use by heterogeneous thin or thick client platforms (e.g., mobile phones, tablets, laptops, and workstations).

Resource pooling  The provider’s computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and reassigned according to consumer demand.

Rapid elasticity  Capabilities can be elastically provisioned and released, in some cases automatically, to scale rapidly outward and inward commensurate with demand. To the consumer, the capabilities available for provisioning often appear unlimited and can be appropriated in any quantity at any time.

Measured service  Cloud systems automatically control and optimize resource use by leveraging a metering capability at some level of abstraction appropriate to the type of service (e.g., storage, processing, bandwidth, and active user accounts). Resource usage can be monitored, controlled, and reported, providing transparency for both the provider and consumer of the utilized service.

Services provided by Cloud Computing Providers in different modules are as follows :-

IaaS [Infrastructure as a Service]   Service providers provides you the resource on rent based on usage ,the resourse includes storage, hardware, servers and networking components of your choice and configuration ,they will not be bothered about how and for what purpose you use it they will take the responcibility of housing, running and maintaining it. Its service providers responcibility to make the availability of Infrastructure [VMs and Network] up and running as per SLA [Service level Agreement].

PaaS [Platform as a Service]    Adding Operating system along with some associated services for running user’s existing application or developing and testing new applications in the IaaS [Infrastructure as a Service] makes it as another kind of offer provided by cloud computing as PaaS. The best example is Windows Azure and Google Apps

SaaS [Software as a Service]   Its a model designed in a way to distribute only software and Service Provider has to make it available to customers over network. Application vendor need not to worry about anything related to Hardware and its configuration. Such as E-Mail facility and Web-services .

Hope there will be some clarity about What cloud computing is about !!