Software as a service (SaaS) is the topmost layer, most visible to the end user. Other on-demand services that are less visible to the end user include platform as a service (PaaS) and infrastructure as a service (IaaS).

Software as a service (SaaS) — sometimes referred to as "on-demand software" — is a software delivery model in which software and its associated data are hosted centrally (on the cloud, for example) and are typically accessed by users using a thin client, normally using a web browser over the Internet.