– Kevin Epstein, vice president of marketing at CloudShare (www.cloudshare.com), says:

Cloud computing is the ability to access as much or as little of one or more computing-related resources as you need, at any time, for as long as you need, at any level of the stack, over a network (local or internet). In other words, cloud computing would let you ask for 10 email servers for an hour (or a year), or 100 email accounts for an hour, or storage for 100 gigabytes of email for an hour. All are cloud computing, though at different levels of the stack (PaaS, SaaS, and IaaS). Cloud computing is de facto -not- a single type of hardware, software, or application from one vendor; the point of a cloud is to allow the ad-hoc introduction or removal of various components (so, using the email example, a single -server- might be replaced without impacting end user service).