What is SaaS?
SaaS stands for at least three different "as-a-service" offerings. The original use of the acronym was to refer to Software as a Service. The term has since been used in reference to Storage as a Service and Security as a Service.
Other "as-a-service" offerings include Infrastructure as a Service (IaaS) and Platform as a Service (PaaS). The combination of these two with Software as a Service is sometimes called the SPI model (for SaaS, PaaS, IaaS). The whole collection of things available "as-a-service" is sometimes referred to as XaaS (which is variously said to refer to "X as a service," "anything as a service" or "everything as a service").