Cloud Computing Definition and Uses
Formally defined, cloud computing is provision of on-demand information technology resources, such as computing power, software, storage, networking services, databases, and other hardware infrastructure through the Internet on pay-as-you-go model without the involvement of consumers in the management task. Client organizations only pay for the services procured from the cloud service providers instead of developing and managing their own infrastructure.
Read about Cloud Computing Models and Architecture
Read about the NIST definition of cloud computing.