From NGO Handbook
Could computing is an Internet-based style of computing, whereby shared resources, software, and information are provided to computers and other devices on demand. Cloud computing customers generally do not own the physical infrastructure, instead avoiding capital expenditure by renting usage from a third-party provider. They consume resources as a service and pay only for resources that they use, similar to the use of the electricity grid. For those of you that are using such tools as Google Apps, Salesforce, Facebook, and Office Live, you are using the cloud.
Gartner, Inc. defines cloud computing as “a style of computing in which scalable and elastic IT-enabled capabilities are delivered as a service to external customers using Internet technologies.”
Let me tell you what that really means. Let me break out some of the keywords here.