What is the difference between IaaS, PaaS and SaaS?

Over recent years I've helped to architect solutions that use more and more cloud services and very often see the following terms used:

- IaaS or Infrastructure as a Service
- PaaS or Platform as a Service
- SaaS or Software as a Service

Just from looking at the names of each service types, most technical people can work out what is provided by each. But unfortunately sometimes people are unsure of the differences, particularly where IaaS ends and PaaS starts.

So i thought i’d put together a simple illustration which may help someone looking for a quick overview of the differences.