Infrastructure as a service (IaaS) vs Software as a service (SaaS)

Why they differ

IaaS hands you raw machines to build on; SaaS hands you a finished program to use.

Cloud

The cloud model where you rent bare building blocks - machines, storage and network - and run everything on top yourself.

Formal

A cloud service model in which the provider supplies processing, storage and network resources, while the customer installs and manages the operating system, programs and data, and some network settings.

In plain English

Like renting an empty warehouse - the landlord keeps the walls and roof sound, but the shelves, locks and stock inside are up to you.

In practice

The IT operations manager at an accounting firm rents a virtual machine from a cloud provider for the firm's finance system and must personally install updates, set up the firewall and create user accounts on it.

Why it matters

It gives the most freedom but leaves the most security work with the customer; teams that assume the provider patches their machines leave gaps open.

Software as a service (SaaS)

Cloud

The cloud model where you simply use a finished program over the internet, and the provider runs everything behind it.

Formal

A cloud service model in which the customer uses the provider's programs, usually through a web browser, without managing the servers, operating system or program itself, apart from limited user settings.

In plain English

Like taking a taxi instead of owning a car - you say where to go, and someone else handles the engine, the fuel and the repairs.

In practice

A school moves to an online email and document suite; the school's IT lead no longer runs mail servers but still decides who gets accounts, whether MFA is on and who files may be shared with.

Why it matters

Even when the provider runs everything, the customer still owns its data, its user accounts and its sharing settings - and many breaches in such services start there.

Shared connections

Cloud computing Platform as a service (PaaS)