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."

Public cloud (pay-for-play) services such as Amazon's EC2 and Microsoft's 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 IDC 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 virtualisation or cloud will get them," according to Roger Johnson, who evangelised both in his previous job as a senior IT manager at audio-systems reseller Crutchfield, and does so now as a senior systems engineer at 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 virtualised, 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) internal, private clouds allow a company to weave layers of virtualisation and management software around existing infrastructure to tie servers, storage, networks, data and applications. The goal: Once they're interconnected and virtualised, 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.

What's the difference between a highly virtualised environment and an internal cloud? VMware says an internal cloud should also have a high degree of management automation and offer chargeback capabilities for business units. Private clouds should make managing both information and technology easier, but will blow apart the silos into which most IT organisations have been built over decades, Wolf says. "Right now the server people talk to the server people, not networks or support or anything else," he says. "If everything's virtualised, everything's on ever box, so your job can't be defined according to where the box you're responsible for sits."

2. External Cloud Hosting

External cloud, any IT service maintained by an external service provider and accessed through the Internet, is the best source for both cost effective IT extensibility and of insecurity, mistrust, confusion and the potential for disaster. Among the best known US providers of external cloud services are Rackspace, Terramark, Equinix, AT&T and IBM. The big worry: In a recent Portio Research survey, 68 percent of respondents say worries about security are holding them back from cloud projects. 58 percent say performance is also a drawback.

"In the public cloud a lot of the fear factor is that your data is sitting on someone else's infrastructure," says Vince DiMemmo, general manager of cloud and IT services at data centre hosting and services company Equinix. "When you hire someone else your expectations for security are much higher, so most customers aren't comparing what a service provider offers compared to what they do in their own systems. They tend to be nervous about cloud, too, not for [co-location] and server hosting that they've been doing for a long time."

There's not a lot of difference between co-location or hosting and cloud services in the platform-as-a-service market, which means any IT organisation with external providers has already done most of the vetting needed for a cloud provider, says Jim Levesque, systems programmer and supervisor of the cloud-based disaster recovery and backup system built by the Los Angeles Department of Water and Power for its 600-server business application network.

"You check the security, make sure about their finances so they're not going to disappear right away, talk to their references and make sure they've got good provisioning on the things that are important, plenty of I/O and network access, redundant connections and power supplies, emergency plans, all that stuff," Levesque says.

But many customers are also worried they'll get locked in to a single service company if the APIs, systems and interfaces their cloud provider uses don't allow them to pick up an move back to internal servers or to a different provider's infrastructure, according to Karl MacDonald, chief evangelist for cloud service provider

3. Hybrid or Modular Approach

It's pretty clear that the near future of IT is the hybrid cloud model, Wolf says. Hybrid cloud computing can include a mix on internal clouds, external cloud services and traditional SaaS options. The mix of pieces that hybrid should include for a specific business will end up being as unique as the IT organisations that provide it, he says.

Some small- and mid-sized companies face the same dilemma as that guy who insists he can wear the same jeans he did in high school, despite their 32-inch waist and his 42-inch belly. The CEO keeps cinching the budget a little tighter every quarter.

Smaller-scale workspace on demand services can fit the bill here. Originally conceived for applications such as on-demand test and development environments (where the need for 100 virtual workstations on which to test a software distribution script wouldn't be unusual) workspace on demand companies such as CloudShare, Soonr or Microsoft Azure offer mini versions of the macro cloud.

Rather than buy large-scale services with a lot of configuration and management from Amazon or other hosting companies, it makes sense to have a service you can use to get IT-on-demand for workgroups rather than enterprises, according to Steve Peltzman, CIO of New York's Museum of Modern Art.

"We, like a lot of companies, have only one set of staging servers for anything, and you don't want to add a feature because you don't want to mess with the staging, so you have to put that off," Peltzman says. "There are lots of needs, strategic or tactical, we have to meet during the day without having a rack of servers to pull out to do it. We look at where it makes sense to outsource to SaaS providers, SalesForce, outsourcing email to Gmail, Amazon or Cloudshare for platform. Sometimes I don't know what we're going to use a specific service or function for, but I know we're going to need it. That's what I'm looking to the cloud for."

4. Traditional SaaS

For those looking for an even smaller slice of additional functionality or capacity, plain jane SaaS may be the way to go. The quickest way to get into "cloud computing" is to sign up for free email at Yahoo or Google, or for productivity apps from Zoho, 37Signals or a host of other services aimed at businesses or individuals.

Google's corporate email is popular among small companies that put managing their own Exchange servers somewhere down below housekeeping and maintenance. Productivity apps are available online from Microsoft, Zoho and others who'd rather not pay for bulk upgrades of feature-heavy desktop applications.

Companies that don't even want to have to maintain Windows can go to Desktone, ThinkGrid and a few other VDI-on-demand providers.