What is "the cloud"? Someone else's computer, explained

A simple definition

When someone says a file is "in the cloud", they mean it is stored on a server, a computer in a data centre somewhere, rather than only on your own phone or laptop. You reach it over the internet.

The cloud is not in the sky. It is rows of real computers in large, secure buildings. A popular joke sums it up well: the cloud is just someone else's computer. That is broadly true, although it is a very large, well-run computer that many people share. For more about these machines, see What is a server?

An everyday comparison

Think about electricity. Most homes do not have their own generator. Instead, you plug into the grid and pay for what you use. Someone else builds and runs the power stations.

The cloud works the same way for computing. Instead of buying and looking after your own powerful computers, you use a provider's machines and pay, if at all, for what you use. A small business can rent server space by the hour. A family can pay a small monthly fee for extra photo storage.

Why people use it

• Access from anywhere. Write a document on your laptop, then read it on your phone on the bus.
• Backups. If your phone is lost or broken, your photos and contacts can be restored to a new one.
• Sharing. Send a link to a folder instead of attaching large files to an email.
• No hardware to look after. The provider replaces failed disks, adds security and keeps things running.
• Grows with you. Need more space? Change your plan instead of buying a new drive.

The downsides to know about

• You need the internet. Without a connection, you may not be able to open files that are only in the cloud. Many apps let you mark important files to keep "offline" as well.
• You depend on the company. If a service closes, changes its prices or locks your account, getting your data back can be hard.
• Your password is the front door. If someone gets your password, they can reach everything stored there, from anywhere.
• Syncing is not the same as backing up. Many cloud folders simply mirror your device. Delete a file on one, and it may disappear from all of them. Check whether your service keeps deleted items for a while, and for how long.

Who is responsible for what?

The provider looks after the buildings, the hardware and the core software. You are still responsible for how you use it: who you share files with, how strong your password is, and whether two-step sign-in is turned on. Businesses call this a shared responsibility. It applies at home too.

Tip: Turn on two-step verification for your main cloud account, the one that holds your email and photos. Then, even if your password leaks, a stranger cannot get in without your phone or security key.

Using the cloud sensibly: a short checklist

1. Find out which cloud service your phone already uses for photos and backups, and check how much space is left.
2. Use a strong, unique password for that account and turn on two-step verification. The password generator can help.
3. Review shared links now and then, and remove access for people who no longer need it.
4. Keep a second copy of anything irreplaceable, such as family photos, on an external drive or a second service.
5. Be wary of emails saying your storage is full and asking you to log in through a link. Open the app yourself instead; this is a very common scam.

Want the technical version? See What is cloud computing? and The shared responsibility model.