The Software as a Service (SaaS) Model is the combination of a Business Model and a Software Delivery Model.
An Independent Software Vendor embracing the SaaS business model, sets up the business in such a way that the key business processes of Marketing, Sales, Billing etc are integrated with the service itself.
An ISV embracing the SaaS Delivery model, sets up the delivery organization in such a way, that the key technical processes of Provisioning and Support are also integrated with the Service itself.
The SaaS Model is therefore more than simply delivering an application over the Internet, it is a whole business mentality, requiring highly automated business processes and sophisticated support engines in addition to the application itself.