So, what is SaaS anyway?

I find this question coming up on a regular basis these days - as always, Wikipedia has an excellent write-up -

 "Software as a service (SaaS) is a model of software delivery where the software company provides maintenance, daily technical operation, and support for the software provided to their client. SaaS is a model of software delivery rather than a market segment; it assumes the software is delivered over the internet. Software can be delivered using this method to any market segment including home consumers, small business, medium and large business."

That is as good a definition as you need IMHO.