I came across an interesting post, Is Virtualization the Same as Cloud Computing?, published by Rajan Chandras. He, in turn, was commenting on an article that equated cloud computing and virtualization. He's arguing that it is incorrect to equate virtualization to cloud computing.
I believe that both Rojan and I would agree that virtualization is abstracting computing functions into a logical environment that appears ideal for that function that may be very different than the actual physical environment. This trades computing and other resources for flexibility, scalability, performance, reliability or some other requirement. In my view, this breaks down to seven layers of technology, each of which contains several different segments. (see Sorting out the different layers of virtualization for more information on my model.)
Cloud computing, on the other hand, is a delivery and consumption model allowing organizations to purchase access to applications, development and deployment platforms and either virtual or physical servers as a service and on a by-the-use model. (Click here to visit the 451 Group's website to download the Cloud Codex which describes our definition of cloud computing.)
While virtualization technology is likely to be in use where the computing services originate, but it is not absolutely required.
Saying cloud computing equals virtualization is a bit like saying all cars equal fuel injection systems because some vehicles use fuel injection as a way to deliver fuel to the engine.