IaaS vs PaaS vs SaaS: Cloud Service Types Explained

A cloud service type describes how the responsibility for running technology is split between you and the cloud provider. Every application depends on a stack of layers: physical hardware, networking, storage, the virtualization host, the operating system, the runtime and middleware, and finally the application and its data. In a traditional on-premises setup, you own and manage every layer yourself. Cloud service types let the provider take over the lower layers so you can focus on what matters to you.

There are three main types, and they form a ladder. Infrastructure as a Service (IaaS) hands you the lowest layers as a managed service but leaves the operating system and everything above it to you. Platform as a Service (PaaS) adds the operating system, runtime, and scaling, so you manage only your application and data. Software as a Service (SaaS) delivers finished software where the provider runs nearly everything and you simply use it.

The single idea to hold in your head: moving from IaaS to PaaS to SaaS, the provider does more and you do less. Choosing a type is really about deciding how much control you want versus how much work you want to hand off. The exam tests whether you can place a service, a use case, or a responsibility on this ladder.

Infrastructure as a Service (IaaS) rents you fundamental computing resources over the cloud: virtual machines, storage, and networking. The provider runs the physical datacenter, servers, and virtualization host, so you never buy or maintain hardware. Everything from the operating system upward is your responsibility. That means you install and patch the operating system, configure it, install runtimes and middleware, deploy your applications, and manage your data.

IaaS gives you the most control of the three service types, and with that control comes the most work. You decide exactly how the machine is sized, which operating system it runs, and how it is secured at the OS level. This makes IaaS feel closest to running your own servers, minus the physical hardware.

A helpful memory hook: if a question says "you manage the operating system" or "lift and shift" an existing server to the cloud, the answer is IaaS. You are renting the raw infrastructure and doing the rest.

A concrete example of IaaS is Azure Virtual Machines, where you provision a virtual server, choose its operating system, and manage it yourself. Because you control the OS, IaaS suits workloads that need specific configurations or that were built to run on a traditional server.

Platform as a Service (PaaS) provides a ready-made environment for building, deploying, and running applications. The provider manages the infrastructure and the operating system, runtime, patching, and much of the scaling. You manage your application code and your data, and little else. You do not log in to patch the operating system, because keeping the platform current is the provider's job.

This is the middle rung of the ladder. Compared with IaaS, you give up some low-level control in exchange for far less maintenance. Developers can focus on writing and shipping software instead of caring for servers. PaaS often includes built-in scaling, load balancing, and development tools, which speeds up delivery.

The exam trigger to remember: if the provider manages the operating system and you deploy your app onto it, that is PaaS. You bring the application; the platform is already prepared.

Concrete examples of PaaS include Azure App Service, a managed platform for hosting web applications and APIs, and Azure SQL Database, a managed database where you work with your data while the provider handles the underlying servers, operating system, and patching. PaaS suits teams that want to build applications quickly without managing infrastructure.

Software as a Service (SaaS) is finished, ready-to-use software delivered over the internet. The provider manages almost the entire stack: hardware, operating system, runtime, the application itself, and most of the data platform. You just sign in and use it. Your main responsibilities shrink to your own data, your user accounts and access, and how you configure the settings the software exposes.

SaaS is the top rung of the ladder and offers the least control but the least effort. There is nothing to install or patch, no servers to size, and no runtime to maintain. You typically pay a subscription and access the software through a browser or app. This makes SaaS ideal when you need working software right away and have no desire to build or host anything.

The exam trigger: if it is ready-to-use software you simply sign in to, it is SaaS. You are a consumer of the application, not a builder or an operator of it.

Concrete examples of SaaS include Microsoft 365 for email, documents, and collaboration, and Dynamics 365 for business applications such as customer relationship management (CRM). Common SaaS use cases are email, CRM, and other everyday productivity tools that many organizations simply want to use rather than run.

The exam's favorite service-type question is really one question in disguise: who manages the operating system? Answer that, and you have usually identified the service type. The table below compares the three types across responsibility and typical use cases. Read "Provider" as the cloud provider managing that layer and "You" as the customer managing it.

Notice the steady pattern down each column: the provider's responsibility grows from IaaS to PaaS to SaaS while yours shrinks. Your data is always partly your responsibility, no matter the type. If you can recall which layers the provider owns for each type, you can answer almost any service-type question on the exam.

Picking a service type comes down to how much control you need and how much you want to manage. Here is how the common use cases map to each type.

Choose IaaS when you need full control over the operating system and configuration. Classic cases are lift and shift (moving an existing on-premises server to the cloud with few changes), running specialized or legacy software that needs a particular OS setup, and dev/test environments you spin up and tear down quickly. IaaS is the answer whenever the requirement is maximum control or OS-level access.

Choose PaaS when you want to build and run applications without managing servers. If a team of developers needs to ship a web app or API and does not want to patch operating systems or configure scaling, PaaS lets them focus on code. The trigger is "build apps without managing infrastructure."

Choose SaaS when you need working software with no building or hosting at all. If the requirement is email, collaboration, or a CRM system that staff can just sign in and use, SaaS is the fit. The trigger is "ready-to-use software."

A quick rule for the exam: more control needed points toward IaaS, less management wanted points toward SaaS, and building your own application in a managed environment points toward PaaS.

Service-type questions often describe a situation and ask you to name the type. Work through these examples using the who-manages-the-OS test.

Scenario 1. A company wants to move its existing on-premises application to the cloud with minimal changes and keep full control of the operating system so it can install custom software. Which type? This is a lift-and-shift with OS control, so the answer is IaaS. A virtual machine gives them the same OS-level control they had before.

Scenario 2. A development team is building a new web application and wants to deploy their code without patching servers or managing operating systems. Which type? The provider handles the OS and runtime while the team manages only the app, so the answer is PaaS.

Scenario 3. A business needs email and a customer relationship management (CRM) system that employees can sign in to and use immediately, with nothing to install or maintain. Which type? This is ready-to-use software, so the answer is SaaS.

Scenario 4. A team needs a database but does not want to manage the underlying server or apply operating-system patches. Which type? A managed database is a platform service, so the answer is PaaS. Notice how each answer follows from a single question: how much are you willing to manage?