Cloud Computing Service: The SaaS

Introduce the SaaS (Software as a Service) in general. Basically, what is SaaS? What are the differences between SaaS and other types of cloud computing services (PaaS, IaaS). What is the current development of SaaS and other interesting and important aspects about SaaS nowadays. Please do not focus on one particular country, especially on U.S. Stay general and global. I do not need any introduction or conclusion parts because that should be just a part of the longer paper, the introduction. Just pure facts listed. Please stay within the topic because those are already the topics of my other classmates: SaaS global market. Market research. Structure, market shares, distribution among the countries, trends, SaaS providers from different industries, from different countries and criteria of comparison.