Jeff Kaplan tries to clarify where he sees the intersect of cloud computing and SaaS. His post is in response to many people using the two terms pretty much interchangeably. Jeff says that;
In my case, I view cloud computing as a broad array of web-based services aimed at allowing users to obtain a wide range of functional capabilities on a ‘pay-as-you-go’ basis that previously required tremendous hardware/software investments and professional skills to acquire. Cloud computing is the realization of the earlier ideals of utility computing without the technical complexities or complicated deployment worries. With this precept in mind, I see SaaS as a subset or segment of the cloud computing market.
One of Jeff’s commenter’s puts a more simple spin on it when he says that;
[the] primary difference between cloud and SaaS is who they serve. Cloud computing serves developers and companies who develop software and services. SaaS serves end users who use software.
So from this definition we see that SaaS is one consumer facing usage of cloud computing. While it’s something of a semantic discussion it is important for people inside to have an understanding of what it all means. Put simply cloud computing is the infrastructural paradigm shift that enables the ascension of SaaS.