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”. SaaS is typically accessed by users using a thin client via a web browser. SaaS is one of three main categories of cloud computing, alongside infrastructure as a service (IaaS) and platform as a service (PaaS). The services of e-invoicing service providers are often self-described as cloud solutions, SaaS or PaaS.