In SaaS products, instead of building the same core ingredient over and over again for multiple companies, it is built once and then made available via the web to anybody who wants to use it.
That's what ViaWeb was, it's what WePay is, it's what Salesforce is -- it's what the web has become. You need software to do something, you Google it, you find a web-app and you start using it. Done.
Web Objects, Web Apps, tomato, tomatoe.