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.