What is IaaS, PaaS, SaaS?

IaaS : Infrastructure as a Service
eg: Amazon EC2, Rackspace, Verizon Private Cloud.
PaaS : Platform as a Service
eg: App Engine, Azure.
SaaS : Software as a Service
eg: Google Docs, Acrobat.com.

Here is a collection of slideshows about IaaS, Paas and SaaS.