Much of cloud computing terminology is based on the notion of ‘as a Service’ (or ‘aaS’).
The ‘as a Service’ tag has migrated to several new uses. Here is my attempt at a set of definitions (and please comment if you disagree):
- SaaS (Software as a Service) – I mainly see this as an application that runs in the cloud and requires the user to download no (or very little, maybe a browser plugin) software to use the application. (e.g. SalesForce, Cisco WebEx, Google Apps)
- DaaS (Data as a Service)* – This is providing data over the cloud either as the result of a query (is the email address me@acme.com valid) or involving a data transformation (correct the address 101 First Ave, Mytown, NC 2513). (e.g. StrikeIron!)
- PaaS (Platform as a Service) - Providing a platform for running applications, data storage abstraction, etc. One step up the software stack then IaaS (e.g. Google App Engine, Force.com/Heroku, PHP Fog)
- IaaS (Infrastructure as a Service) – Providing a virtual machine and storage mechanisms that can be loaded with operating systems and software (custom, open source, commercial, etc). (e.g. Rackspace, Amazon AWS, GoGrid)

There are some proprietary aaS’s as well. My favorite is HP’s Everything as a Service. I am not sure what this really is but it sounds impressive.
Clear as mud? There is certainly some overlap between the different technologies but at the end the trend is clear. Leverage the efficiencies of scale, lower the barrier of entry, and speed up the time for implementation.
*DaaS can also refer to “Desktop as a Service” and “Database as a Service” in several sources.