SAAS, PAAS and IAAS : Three ways to Cloud Computing

There typically three ways to cloud computing. People often hear about these three (PAAS, SAAS, IAAS) terms in cloud computing.

 Lets discuss about these three terms here. Lets know what they are, what are their pros and cons. Firstly, abbreviations 
SAAS : Software as-a service 
PAAS : Platform as-a service
 IAAS : Infrastructure as-a service 
SAAS Software as-a service Allow users to run existing online applications Where off-the-shelf applications are accessed over the internet Examples: microsoft, google and zoho presentation and spreadsheet packages 
PAAS Platform as-a service Allows users to create their own cloud applications using supplier specific tools and languages Provides environment and tools for creating new online applications Examples: Google API engine Azure microsoft Salesforce.com IAAS Allows users to run any applications they please on cloud hardware of their own choice Allows existing applications to be run on a cloud suppliers hardware Virtualization