Most agree that cloud computing is the Next Big Thing, but beyond that things get murky. Being such a new space means that there’s not yet a consensus on what all the pieces are, and how they fit together.
Michael Crandell gives a good descrption of what he considers to be the three tiers of cloud computing: apps, platforms, and infrastructure. (He correctly puts Heroku on the middle tier.)
It gets a bit harder as you try to subdivide each tier. This diagram is one I draw a lot these days, but it’s a little different each time, as we continue to discover new challenges about our slice of the cloud computing pie.