Cloud Computing is another buzz-word of this time, but its meaning is fuzzy (e.g. like the Web 2.0), ranging from pure virtualization to * as a service.
This article gives 20 definitions from various experts.
I like this view of Michael Sheehan very much which gives some structure to this range:
"I would like to propose a 'Cloud Pyramid' to help differentiate the various Cloud offerings out there. [At the top of the pyramind] users are truly restricted to only what the application is and can do. Some of the notable companies here are the public email providers (Gmail, Hotmail, Quicken Online, etc.). Almost any Software as a Service (SaaS) provider can be lumped into this group.
As you move further down the pyramid, you gain increased flexibility and control but your a still fairly restricted to what you can and cannot do. Within this Category things get more complicated to achieve. Products and companies like Google App Engine, Heroku, Mosso, Engine Yard, Joyent or force.com (SalesForce platform) fall into this segment.
At the bottom of the pyramid are the infrastructure providers like Amazon’s EC2, GoGrid, RightScale and Linode. Companies providing infrastructure enable Cloud Platforms and Cloud Applications. Most companies within this segment operate their own infrastructure, allowing them to provide more features, services and control than others within the pyramid."
And from IBM's Irving Wladawsky Berger:
"When virtualizing applications to be used by people who care nothing about computers or technology - as is mostly the case with Clouds - the key thing we want to virtualize or hide from the user is complexity. Most people want to deal with an application or a service, not software. ... The more intelligent we want [computers and computer applications] to be - that is, intuitive, exhibiting common sense and not making us have to constantly take care of them - the more smart software it will take. But with cloud computing, our expectation is that all that software will be virtualized or hidden from us and taken care of by systems and/or professionals that are somewhere else - out there in The Cloud."