What the cloud is

The cloud is computing you rent over the internet instead of owning. You ask for a server, storage or a database, it appears in minutes, and you pay for what you use. When you no longer need it, you delete it and stop paying.

The US standards body NIST gives the classic definition: on-demand self-service, broad network access, shared pools of resources, rapid elasticity, and measured service. In short: you serve yourself, it scales, and it is metered.

Infrastructure as a service, IaaS, rents you basic building blocks such as virtual machines, disks and networks. You manage the operating system and everything on it. Platform as a service, PaaS, runs your code or database for you, so you manage less. Software as a service, SaaS, is a finished app such as email or a CRM that you simply use.

The further you move from IaaS towards SaaS, the less you control and the less you have to look after.

Providers run data centres around the world grouped into regions, such as one in Singapore or Frankfurt. Each region has several availability zones, separate buildings with their own power and networking. Spreading an app across zones keeps it running if one fails.

Choose a region close to your users for speed, and check the law: some data must stay in a particular country.

The provider secures the cloud itself: buildings, hardware and the core platform. You secure what you put in it: your data, who has access, and how you configure services. Most cloud breaches come from the customer's side, such as a storage bucket left public or a leaked key. The exact split shifts with the service type, and every major provider publishes its version.