Sreedhar Kajeepeta, CTO of Computer Sciences Corp., has written an column for Computerworld explaining the concept of multi-tenancy.

That concept really isn't all that much. It just constitutes the core of the whole cloud computing model.

While I can reproduce the article briefly in my own words, I don't think that's necessary. It is a quick read, comprehensible and it offers some examples. You can read it here.