Anybody with even a vague understanding of Cloud Computing knows it comes in three basic flavors: Infrastructure-as-a-Service (IaaS), Platform-as-a-Service (PaaS), and Software-as-a-Service (SaaS). If you press a general audience to further define these Cloud Service Models, they may give you answers like, "IaaS? You know, like Amazon?" and "SaaS? That's like Salesforce.com, right?" PaaS is not so lucky: "PaaS? Development tools in the Cloud, right?" is about as close as anyone is likely to get.
While each of these three core Service Models does have a clear definition (see pages 2 -- 3 of the NIST Definition of Cloud Computing in particular), there remains substantial confusion in the Cloud marketplace about the specifics of each model.
In the case of IaaS and SaaS, it's true that Amazon and Salesforce.com essentially defined the respective models, for better or worse. Sure, it's great to be able to point to prototypical IaaS and SaaS offerings, but on the downside, these two companies are driving their markets to help their businesses, not out of some altruistic urge to clear up confusion.
The end result is that there are many Cloud offerings that don't fit neatly into the three Service Models, either because they're not sufficiently like Amazon's IaaS or Salesforce.com's SaaS, or alternatively, because the vendor in question is looking to differentiate themselves from the three models altogether. This push for differentiation has led to a plethora of alternative Service Models, including Governance-as-a-Service, Data-as-a-Service, Identity-as-a-Service, Mobile-Backend-as-a-Service, and many others. Some of these new *aaSes may be subcategories of the three main Service Models, but in most cases, the vendor is looking to define and differentiate their own Service Model.
In other cases the new *aaS is in a gray area between two Service Models, for example, Database-as-a-Service. Is it PaaS or IaaS? Bottom line: it doesn't really matter. The marketplace is responsible for defining the Service Models. In other words, we can make up names all we like, but it's what people want to buy that drives market categorization. Today's three Service Models are a convenient categorization of an emerging marketplace. But only the marketplace can decide the Service Models of the future.
PaaS, IaaS, daas