IaaS vs PaaS vs SaaS Explained

This is the single most foundational concept on the AZ-900 exam, and nearly every service-selection question implicitly depends on understanding it. The three service models describe a sliding scale of what you manage versus what Microsoft manages.

Infrastructure as a Service (IaaS) gives you the most control and the most responsibility. Microsoft manages the physical hardware, networking, and virtualisation layer; you manage everything from the operating system upward — patching, runtime, applications, and data. Azure Virtual Machines are the textbook IaaS example.

Platform as a Service (PaaS) removes the operating system layer from your responsibility. Microsoft manages the infrastructure and the platform — OS patching, runtime, middleware — while you manage your application code and data. Azure App Service is the classic PaaS example: you deploy your code, Microsoft handles everything underneath it.

Software as a Service (SaaS) removes nearly everything from your responsibility except your data, your devices, and your user accounts. Microsoft manages the entire stack — infrastructure, platform, and the application itself. Microsoft 365 is the standard SaaS example.

A simple way to remember the ladder: in IaaS you manage from the OS up; in PaaS you manage your app and data; in SaaS you manage only your data, devices, and identities. Those three responsibilities — data, devices, identities — never transfer to the provider in any model.

The exam often tests this indirectly, through a scenario describing what a company wants to control or avoid managing, then asking which model fits. If the scenario says a company doesn't want to manage any infrastructure and just wants to deploy code, that's PaaS. If it says a company needs full control over the operating system for a legacy application, that's IaaS.