Cloud computing refers to a style of computing in which resources are provided as a service over the Internet to users who need not have knowledge of, expertise in, or control over the technology infrastructure. The canonical example is Google Docs, a functional word processing office app delivered entirely via one's web browser . The level of interest in Cloud Computing as an accessible service for the realisation of new web based applications has been growing over the past year with both academic research  and is also leaking into mainstream discussion . Discussion on how the cloud will impact high performance computing  and other areas such as its use in Green IT are also ongoing.
Quote: "Cloud Computing, the long-held dream of computing as a utility, has the potential to transform a large part of the IT industry, making software even more attractive as a service and shaping the way IT hardware is designed and purchased. Developers with innovative ideas for new Internet services no longer require the large capital outlays in hardware to deploy their service or the human expense to operate it. They need not be concerned about overprovisioning for a service whose popularity does not meet their predictions, thus wasting costly resources, or underprovisioning for one that becomes wildly popular, thus missing potential customers and revenue. Moreover, companies with large batch-oriented tasks can get results as quickly as their programs can scale, since using 1000 servers for one hour costs no more than using one server for 1000 hours. This elasticity of resources, without paying a premium for large scale, is unprecedented in the history of IT." 
Some of the different application areas where cloud computing might make an impact include :
UCSB & IBM T.J. Watson Research five-layer grouping.
Discussions on cloud ontologies have come to light that provide a great starting point for taxonomy discussions .