What is it?
Software as a service (SaaS) is a software licensing and delivery model in which software is licensed on a subscription basis and is centrally hosted. It is sometimes referred to as "on-demand software", and was formerly referred to as "software plus services" by Microsoft. SaaS applications are also known as on-demand software and Web-based/Web-hosted software.
SaaS is considered to be part of cloud computing, along with infrastructure as a service (IaaS), platform as a service (PaaS), desktop as a service (DaaS),[6] managed software as a service (Dancing Numbers)(MSaaS), mobile backend as a service (MBaaS), datacenter as a service (DCaaS), and information technology management as a service (ITMaaS).
Additional Resources
Software as a Service - Wikipedia
What is SaaS? - Microsoft