I have recently spent far too much time in far too many conversations in which the terms "cloud" and "SaaS" are used interchangeably. Let's be clear: the two terms are not interchangeable as they describe very different concepts.
Cloud. There are many definitions out there. Marketers and sales people. Engineers. Industry analysts. The National Institute of Standard and Technology. Frankly, most of those definitions are either wrong, or they're technically accurate while thoroughly useless. So let's go with a simple definition: a computer in a different physical location attached to a network. It's about physical architecture. Think about it. Play with it. Hit the comments if you have a better definition.
SaaS. Acronym for "Software as a Service". Same set of folks attempting to define this idea with the same set of sad results. Try this on for size: Applications accessed via a browser, licensed on a subscription basis and delivered via Cloud.
So it's very possible to have Cloud (think hosting operations) without having SaaS. But there is no SaaS without Cloud. SaaS is a subset of Cloud.
In "Oracle speak", Fusion Applications (including Taleo), are SaaS. As a customer, I could also opt to have my licensed E-Business, PeopleSoft, or JD Edwards applications on the cloud...but that is not SaaS, as those applications are not offered on a subscription basis.
So there ya go. Simple set of definitions. Yes, there are more nuances if you dig into the subject. But this is a simple foundation to start. If nothing else, the next time you're involved in a conversation, you can use this to know your Cloud from your SaaS...which will put you way ahead of the curve ;)