Cloud computing defies one definition, so here are a few of the latest

In some ways, cloud computing is nothing new. IBM, EDS, Hewlett-Packard Co. and Microsoft have talked for years about a utility-based computing model that makes IT as easy to use and as reliable as picking up the phone. There would be no disruptions in service because the IT infrastructure behind the service is automated to swap out, crank up or turn down a system as an organization's or subscriber's needs change.

What is new is that many of those promises are becoming a reality. Just look at the popularity of Software as a Service or SaaS, the poster child being Salesforce.com Inc., with its 59,300 customers and its own development platform. Businesses sign up for its service, just like they would a cell phone agreement; the service works, and they pay as they go.

Still, there are many vendors jumping on the cloud computing bandwagon, just as they did with virtualization and before that with the "it" term of the late 90s: ASPs. Salesforce.com was once called an ASP, or application service provider; eventually it became known as a SaaS provider. Some now even consider it a Platform as a Service or PaaS provider due to its Force.com development platform. This is not to pick on Salesforce.com and the value that customers derive from its applications and development platform. It's just an example of how technology terminology changes for the services that software and hardware vendors provide.

The term ASP isn't cropping up this time around, but many others are, such as SIaaS, HIaaS and PaaS. But before we get to those definitions, let's start with a few at the top.

Cloud computing definition, take one

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on Group Inc. in Midvale, Utah, offers a basic definition for cloud computing: a set of disciplines, technology and business models used to deliver IT capabilities (software, platforms, hardware) as an on-demand, scalable, elastic service.

Gartner Inc. in Stamford, Conn., defines cloud computing as "a style of computing where scalable and elastic IT capabilities are provided as a service to multiple customers using Internet technologies."

The cloud computing definition on our sister site, SearchCloudComputing.com, begins, "Cloud computing is a general term for anything that involves delivering hosted services over the Internet." Wikipedia's cloud computing definition is similar.

These definitions then dive into a layer of acronyms, including some or all of the following:

• SaaS. This is a software delivery model in which a vendor licenses an application to customers on a pay-as-you-use basis, versus buying a traditional yearly or several-year license. SaaS vendors maintain and update the application and host it for the customer, either on their own (the vendor's) infrastructure or through a third-party data center.

  New SaaS providers pop up every week, but examples of popular SaaS players and applications include Google Apps, online email, Salesforce.com, SaaS development tools and Data as a Service, according to the Burton Group.

• PaaS is a development platform that customers or developers can use to develop applications and services for use in the cloud. Examples include Microsoft's Windows Azure, the Google App Engine, Salesforce.com's Force.com, LongJump, TIBCO Silver and Amazon Web Services, said Burton analysts.

• SIaaS, or Software Infrastructure as a Service, is a standalone service that provides a specific application support capability such as identity management or a content distribution system, according to Burton analyst Chris Haddad. "SIaaS will target a specific aspect of the application software platform," Haddad said during the Burton Group recent Catalyst show in San Diego.

  SIaaS comes in many shapes and sizes, he said, such as database services like Amazon SimpleDB, Microsoft SQL Azure and data distribution services like those from Akamai Technologies Inc., Amazon CloudFront, Level3 Communications LLC and Limelight Networks Inc.

  And other services will emerge in this space such as identity and security services from the likes of Novell Inc., Microsoft and RSA Security Inc., he said.

• HIaaS, or Hardware Infrastructure as a Service (also called Infrastructure as a Service (IaaS) or even Hardware as a Service) provides infrastructure components on demand. These might be compute power, storage or network pieces. Think Amazon Web Services -- including its Elastic Compute Cloud, or EC2, Joyent Inc., Rackspace Hosting Inc. and system hosting providers such as AT&T, Sprint and BT, Haddad said.

And if these definitions don't quite fit with how you have heard the term cloud computing used, Cloud Computing Journal offers 21 other cloud computing definitions.

Let us know about your definition of cloud; email: Christina Torode, Senior News Writer