CIO — Cloud computing is famous for being a metaphor instead of a technology, but that metaphor is increasingly hard for non-techies to understand. Many variations of cloud have emerged that have little to do with the initial vision that sparked interest— a public cloud with burst-up capability on demand.

"Public cloud is not what most of our clients are talking about right now," according to Chris Wolf, analyst for Gartner Group's Burton Group consultancy. "Pretty much everything's hybrid."

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Public cloud (pay-for-play) services such as Amazon's EC2 and Microsoft's (MSFT) Azure were the proof-of-concept for cloud technology. Rather than shift the majority of their own IT to professionally maintained shared-resource services such as those, however, most companies are today using cloud to build on their internal virtual infrastructures, analysts say.

The greatest benefit of cloud is its ability to connect otherwise incompatible infrastructures, not just one or two applications at a time, and its ability to let customers dial up more compute power when they need it, says International Data Corp. analyst Ian Song. Nevertheless, IDC's market surveys predict that spending on cloud will rise from $17 billion in 2009 to $44 billion in 2013.

"It's not real clear in most people's minds what virtualization or cloud will get them," according to Roger Johnson, who evangelized both in his previous job as a senior IT manager at audio-systems reseller Crutchfield Corp., and does so now as a senior systems engineer at Richmond, Va.-based integrator SyCom Technologies.

"Most people seem like they're interested in cloud but they don't want to touch it until there's more adoption and a better track record," says Johnson.

Most companies take a roll-your-own approach to cloud, adding cloudlike interfaces to existing systems, building new systems on virtualized, highly interoperable systems, or hiring co-location, server hosting or online services to meet specific needs or east particular points of pain, Wolf says.

There is no single model for how best to mix all the various cloud service permutations, but a few consistent models have emerged:

1. Internal Clouds

In what's turning out to be the most common form of cloud computing (and convenient for virtual-server vendor VMware, (VMW)) internal, private clouds allow a company to weave layers of virtualization and management software around existing infrastructure to tie servers, storage, networks, data and applications. The goal: Once they're interconnected and virtualized, IT can shift storage, compute power or other resources invisibly from one place to another to give all the end-user divisions all the resources they need at any time, but no more than that.

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