I was speaking yesterday with someone in charge of the SaaS initiative at a very large technology company. In the conversation he described Software as a Service (SaaS) as a "delivery model". Now of course SaaS is a delivery model, but it is not just a delivery model, nor is it primarily a delivery model.
In fact the word delivery starts from the wrong place. Isn't SaaS mostly about the consumption model? That is, we should start from the customer's perspective, not the vendor's, and the customer wants to consume software like a utility that can be plugged in, configured, even integrated, then used. The usage grows as the demand grows, and often the payments also grow with usage. Therefore the customer does not have to spend money up front on hardware, software, and people, but can pay as they go.