SaaS, PaaS and IaaS are three acronyms that get thrown around as if everyone agrees what they mean. They are simply three levels of how much of the technology stack you rent versus run yourself. SaaS is finished software you just log into. PaaS is a platform you build your own app on without touching the servers. IaaS is raw computing you control almost completely. Once you see them as a ladder of responsibility, the jargon dissolves and choosing the right one becomes obvious. This guide decodes all three with a pizza, a clear comparison, and the questions that actually decide which you need.
One ladder, three rungs
Every piece of cloud computing is really a stack: the building and power, the servers, the operating system, the runtime, the application, and your data on top. The only thing that changes between IaaS, PaaS and SaaS is where the line sits between what the provider runs and what you run. Lower down the stack you take more control and more responsibility; higher up you hand both to the provider and just consume the result.
The classic way to picture it is pizza. Cooking from scratch at home is traditional on-premises IT: your kitchen, your ingredients, your effort. IaaS is a take-and-bake kit: someone supplies the base and toppings, you bake it. PaaS is delivery: the pizza arrives cooked, you supply the table and drinks. SaaS is eating at the restaurant: you turn up and eat, and someone else does absolutely everything else.
IaaS: rent the raw building blocks
Infrastructure as a Service gives you the fundamental ingredients of computing, virtual servers, storage and networking, as an on-demand utility. The provider runs the physical data centre and the virtualisation; you install and manage the operating system, the software and everything above it. It is the most flexible model and the closest to running your own servers, just without owning the hardware or the building.
IaaS suits teams that want control: migrating existing applications as they are, running specialised software, or building a setup that does not fit a ready-made platform. The price of that flexibility is responsibility, because patching, configuration and security above the hypervisor are yours. Under the bonnet IaaS is built on virtualisation, which is what lets a provider carve one physical server into many rentable ones.
- •You manage: operating system, runtime, applications, data and most configuration
- •Provider manages: physical hardware, networking and virtualisation
- •Best for: lift-and-shift migrations, custom stacks and maximum control
PaaS: build without minding the servers
Platform as a Service hands you a ready-made environment to build, run and scale your own applications without managing the servers underneath. The provider looks after the operating system, the runtime and the scaling; you bring your code and your data. For development teams it removes a huge amount of undifferentiated heavy lifting, because nobody has to patch an OS or size a server to ship a feature.
PaaS shines when your value is the application you are creating, not the plumbing beneath it. The trade-off is less low-level control and a closer tie to how that particular platform works, which can make moving elsewhere harder later. It is the natural home for in-house developed software where speed of delivery matters more than fine-grained control of the stack.
SaaS: just log in and use it
Software as a Service is finished software delivered over the internet that you simply subscribe to and use. Microsoft 365, your CRM, your accounting package and your helpdesk tool are almost all SaaS. The provider runs everything, the infrastructure, the platform and the application, and pushes updates automatically. You manage your users, your settings and your data, and little else.
SaaS is why most businesses now consume software rather than install it: no servers to run, predictable subscription cost, and access from anywhere. The things to watch are subscription sprawl, where licences quietly multiply, and the fact that your data lives in someone else's system, which makes your security configuration and exit plan important. Our Microsoft 365 licensing optimisation guide is a good example of keeping SaaS spend honest.
Picking the right rung
Choose by asking how much you want to run yourself and where your value really lies. If you are buying a capability and never want to touch infrastructure, SaaS. If you are building your own application and want to focus on code, PaaS. If you need deep control or are moving existing systems wholesale, IaaS. Most businesses use all three at once: SaaS for everyday tools, PaaS or IaaS where they build or run something bespoke.
These models map directly onto the public, private and hybrid debate, so read them alongside our public vs private vs hybrid explainer. And if part of your answer is running your own infrastructure rather than renting it, our deeper cloud-model breakdown and the server configuration service are the logical next steps.