SaaS

Software as a Service (SaaS) is the delivery model where a software vendor hosts, runs, and updates an application, and customers access it over the internet — typically through a browser — paying a recurring subscription per user. Microsoft 365 is the canonical example: Microsoft runs the servers, applies updates, scales the infrastructure, and provides uptime; customers configure the service, manage their users and data, and consume the apps. SaaS sits above PaaS (platform-as-a-service, like Azure App Service) and IaaS (infrastructure-as-a-service, like Azure VMs) in the cloud-service hierarchy. It shifts effort from "running software" to "configuring software."