Easy to Understand one-word descriptions of each term (i.e. IaaS means host)
Data Visualization of Vendor Management Responsibility of Services
IaaS - the unit you are gaining is a computer/server. Therefore IaaS is essentially a “physical server box”. An example of this would be going to RackSpace or SoftLayer and leasing a physical box from them. The vendor manages the networking, hard drives (if they fail), hardware of the box, virtualization O/S (if the box is virtualized). You can remote desktop to the box and you manage everything else (shown in the screenshot above). Windows Azure provides IaaS in the form the VM Role (you upload a Windows Server 2008 R2 image and manage the server yourself).
PaaS - the unit you are gaining is an application/framework. Therefore PaaS is a “hosted application/framework/tools that you can leverage to build something on. That application is configured on IIS/SQL Server etc and runs on a hardware/virtual system that the vendor manages. An example of PaaS would be Windows Azure (excluding the VM Role) services like web role, worker role, Reporting Services etc.
SaaS - the unit you are gaining is business functionality. For example, Gmail is a type of a SaaS mail provider because you don’t have to manage any service yourself and its all done by the vendor (Google in this example).