What is Cloud Computing?

Recently a group of us formed a "Cloud Computing" mailing list on Google Groups. At last count we had 176 members.

One of members asked an interesting Question, What's the difference between cloud computing and Grid Computing? Below is an excerpt from my attempt to answer him

As for what is cloud computing, I've been asked this a lot lately and I seem to give different answers depending on who is asking. When I'm presenting to a group of VC's it's a fundamental shift in the way people interact with applications, a kind of ubiquitous network computing environment. But then again VC's like that kind of talk, so i won't B-S here.

I typically break up the idea of cloud computing into three camps and this is probably how I will attempt to explain it next week during my presentation at Interop in Las Vegas.
http://unconference.interop.com/wiki/index.php/Main_Page

Enablers - These are companies that enable the underlying infrastructures or the basic building blocks. These companies are typically focused on data center automation and or server virtualization (VMware/EMC,Citrix,BladeLogic, RedHat, Intel, Sun, IBM, Enomalism, etc) These enables can range from the chip level such as intel VT, to the hypervisor such as xen or vmware to the orchestration such as 3tera or our Enomalism elastic computing platform.

Providers - (amazon web services, rackspace, Google, Microsoft). The ones with the budgets and know how to build out global computing environments costing millions or even billions of dollars. Cloud providers typically offer their infrastructure or platform. Frequently these "As a Service" offerings are are billed & consumed on a utility basis.

Consumers - On the other side of the spectrum I see the "consumers" companies that build or improve their web applications on top of existing clouds of computing capacity without the need to invest in data centers or any physical infrastructure. Often these two groups can be one in the same such as Amazon (SQS,SDB,etc), Google (Apps) and Salesforce (Force). But they can also be new startups that provide tools & services that sit on top of the cloud (Cloud management). Cloud consumers can be a fairly broad group including just about any application that is provided via a web based service like a webmail, blog, social network, etc. Cloud computing from the consumer point of view is becoming the only way you build, host and deploy a scalable web applications these days.

As for Grid, I would describe grid as a methodology for the distribution of parallel computing processes. Taking a job (workload) and running it on more then one CPU.