Network World — When "cloud computing" became the buzz-phrase du jour in IT sometime in the past year or two, you could probably have been forgiven for having no idea what those two words meant, or for questioning whether they meant anything at all.

But as a major cloud computing conference in New York City made clear this week, analysts and vendors are converging on a standard definition of cloud computing, and agreeing that the cloud approach to technology is gaining traction in the minds of service providers and customers.

Cloud computing is not really a technology by itself but an approach to building IT services that harnesses several converging factors in the IT world, including the rapidly increasing horsepower of servers and virtualization technologies that unleash power by combining many servers into large computing pools and dividing single servers into multiple virtual machines that can be spun up and powered down at will.

Led by companies such as Amazon, vendors are building massively scalable server farms to offer compute power, storage, business software and application building platforms over the Internet, using self-service interfaces that let customers acquire resources at any time they want and get rid of them the instant they are no longer needed. Private clouds deployed by enterprises for their own users are built along the same principles, but done so completely within the firewall.

Amazon CTO Werner Vogels, delivering a speech at Sys-Con's Cloud Computing Conference & Expo this week, showed a slide featuring an early 1900s beer brewery that contained its own power generators. "They had to be experts in electricity to brew beer. Something is off there," he said. "These guys couldn't wait to dump their own generators and start to use electricity from other companies."

Just as electricity became a shared service, or utility, so too will computing power, Vogels and other commentators say. If you are the founder of a start-up that is building an application for Facebook, you have to prepare for the possibility of becoming immensely popular overnight, Vogels says. But you might also fail. That's why you need on-demand access to the power of 5,000 servers at any time, without having to spend the money up front. Or if you run a seasonal business, you may need huge amounts of computing power one month out of the year, but very little during the remaining 11 months.

"There is a shift from infrastructure being a capital expense to a variable cost," Vogels said..

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