By the end of this lesson you will be able to
- Explain the cloud as computers in data centres that you reach over the internet.
- Compare local and cloud computing on cost, access, speed, control and privacy.
- Describe what availability means and why a cloud copy is not a backup by itself.
- Decide whether a task suits local, cloud or a mix.
You hear about the cloud every day: cloud storage, cloud photos, cloud meetings. The name suggests something vague and weightless, but there is nothing mysterious in it. It is a way of using computers that are not in your house. Understanding this clearly helps you decide where to keep files, how much to trust a service, and what to do when it stops working.
What the cloud actually is
Cloud computing means using computing power, storage or software that runs on someone else's computers, called servers, and reaching it over the Internet. Those servers sit in large buildings called data centres, with strong security, backup power and fast network links. Think of electricity: you do not run a generator at home, you plug in and the power station somewhere else does the work. With the cloud you plug in with a browser or an app.
Because one provider's machines serve millions of customers, resources are shared. A company can use a small slice of a big server and pay only for what it uses. Often a single physical server is divided into several pretend computers, each called a virtual machine, and each customer gets their own as if it were a separate machine.
Read this figure as text
A diagram showing a laptop with local files on one side, and a laptop connected through the internet to a data centre on the other.
Two zones. Left, Local: a laptop holds its own files and runs its own programs, with no internet needed. Right, Cloud: a laptop connects over the internet to a data centre containing servers and storage. Arrows labelled request go from the laptop to the data centre and arrows labelled result come back. If the internet link breaks, the cloud side stops working but the local side carries on.
What the cloud gives you
- Access from anywhere. Sign in from a phone, a library computer or a new laptop and your files are there.
- Sharing and teamwork. Several people can open the same document at once and see each other's changes.
- No big purchase. You rent what you need by the month instead of buying equipment, and you can stop when you no longer need it.
- Someone else does the maintenance. The provider updates, repairs and replaces the machines.
- Room to grow. A small website that suddenly gets many visitors can be given more power quickly.
What it costs you
- You need the internet. Without a connection most cloud services stop working. Some apps keep an offline copy, but only if you set that up beforehand.
- You depend on the provider. If the service has a fault, changes its price or closes your account, you cannot fix it yourself.
- Your data is on their machines. Your files are stored under the provider's rules and can be subject to the law of the country where they are kept. Read the privacy settings.
- Costs add up. Small monthly fees across several services can total more than expected, and free plans often have limits or show adverts.
- Speed depends on your connection. Uploading a large video over a slow link takes a long time.
| Question | Local (your own computer) | Cloud (provider's servers) |
|---|---|---|
| Where is the data? | On your own disk | On the provider's servers, often copied to several places |
| Do I need the internet? | No | Yes, for most things |
| Who maintains it? | You | The provider |
| Who controls it? | You | You and the provider, under their rules |
| Sharing with others | Send a copy; copies can drift apart | Share a link or invite people; one shared copy |
| Cost pattern | Buy once, then repair and replace | Small regular payments or a free plan |
| If the device is lost | Files are gone unless you made a backup | Files remain safe on the service, if you can still sign in |
- Works offline
- You control the data
- No subscription
- You must maintain and back up
- Sharing is awkward
- Access from anywhere
- Easy sharing
- Provider maintains
- Needs internet
- Depends on the provider
- Regular cost
Read this figure as text
Two columns listing the strengths and weak points of local computing and of cloud computing.
Two columns. Local computing: strengths are that it works without internet, you control the data and there is no subscription; weak points are that you maintain it, a lost device loses the files without a backup, and sharing is awkward. Cloud computing: strengths are access from anywhere, easy sharing and provider maintenance; weak points are the need for internet, dependence on the provider and a regular cost.
Availability and backups
Availability (or uptime) is the share of time a service works when you need it. It is written as a percentage. 99 per cent sounds high, but it allows about three and a half days of downtime in a year. Each extra nine cuts the allowed downtime by about ten times. Large providers copy your data to several machines and often several buildings, so one broken disk or one fault in one room does not lose it. That protects against hardware failure; it does not protect against everything.
Read this figure as text
A bar chart of yearly downtime in minutes for 99, 99.9, 99.99 and 99.999 per cent availability.
Four bars on a logarithmic scale showing allowed downtime per year. 99 per cent availability: about 5,256 minutes, roughly 3.7 days. 99.9 per cent: about 526 minutes, roughly 8.8 hours. 99.99 per cent: about 53 minutes. 99.999 per cent: about 5 minutes. Each extra nine cuts the downtime about ten times.
Choosing between local and cloud
It is rarely all or nothing. A sensible mix is common: keep working files on your laptop, sync the ones you need on your phone, share a document through a link, and keep a backup of everything. Ask four questions of any task. Do I need it when I am offline? Do others need it? How private is it? What happens if the service is unavailable tomorrow?
- Needed with no internet?
- YesDo thisKeep local, and back it up
- NoDo others need to use it with you?
- YesDo thisUse cloud sharing, plus a separate backup
- NoHighly sensitive (for example identity papers)?
- YesDo thisKeep local and protected, with a backup
- NoDo thisChoose what is convenient; keep a backup
Read this figure as text
A decision tree asking about offline use, sharing and sensitivity, ending in local, cloud or a mix.
Start: do you need it with no internet? If yes, keep a local copy and back it up. If no, do other people need to use it with you? If yes, use a cloud service and keep a separate backup. If no, is it highly sensitive, such as identity documents? If yes, keep it local and encrypted with a backup. If no, cloud or local both work; choose the more convenient and keep a backup.
Common mistakes
✖ Instead of: Thinking cloud files cannot be lost.
✔ Do this: Accounts get locked, files get deleted and services close. Keep an independent backup of anything important.
✖ Instead of: Treating a synchronised folder as a backup.
✔ Do this: A sync mirrors mistakes. A backup keeps older, separate copies.
✖ Instead of: Using one weak password for the email and cloud accounts.
✔ Do this: Use a strong unique passphrase for each, with multifactor authentication.
✖ Instead of: Assuming everything must go to the cloud.
✔ Do this: Pick per task: offline needs, sharing, privacy and cost all matter.
If something goes wrong
- A cloud service will not load but other websites work.
- The service may be down. Check the provider's official status page or try again later, from another network if you can. Do not enter your password on links from messages claiming to fix it.
- My files are not appearing on my phone.
- Check that you are signed in to the same account on both devices, that sync is switched on, and that the phone has a connection. Large files take time to upload first.
- I am nearly out of cloud storage.
- Find the largest files in the service's storage view, delete or move what you do not need, and empty its bin. Only then consider paying for more space.
- I can no longer sign in to my cloud account.
- Use the provider's official recovery page, typed in by hand, with your recovery codes or backup method. This is why recovery codes should be saved safely before you need them.
Key terms
- Cloud computing
- Using computers and services run by a company over the internet instead of on your own device.
- Availability
- How much of the time a service is working and reachable when you need it.
- Virtual machine
- A pretend computer, run as a program on a real computer, with its own system and files.
- Data centre
- A building full of servers, with strong power, cooling and network links.
- Server
- A computer that holds information or services and sends them to other devices that ask.
Practise it for real
Reading is not the same as being able to do it. Complete these on your own computer, then tick only what you have actually done. These ticks are your own honest record: nothing checks them for you.
Map where your files and services live
Make a three-column list: Only on my device, Only in a cloud service, Both. Put at least eight things in it, such as documents, photos, email, contacts and music. For each cloud item write the name of the service. Mark any item that has no separate backup.
Challenge: what stops working offline?
Pick three tasks you do on your laptop, for example writing a letter, watching a video and checking email. Turn Wi-Fi off from the system menu (top right on Ubuntu, Quick Settings on Windows 11), try each task, note what still works, then turn Wi-Fi back on. Write a sentence for each saying why it did or did not work.
Knowledge check
6 questions. You need 80% to pass. Mistakes here are part of learning, and you can retry as often as you like.
Summary
- The cloud means computers in data centres, reached over the internet.
- Resources are shared and often split into virtual machines.
- Cloud gives access and sharing; local gives control and offline use.
- Availability is the share of time a service works; each extra nine cuts downtime tenfold.
- Sync is not backup. Protect the account with a strong passphrase and multifactor authentication.
Checklist: can you do these?
Marking a lesson as read does not prove you can do it. Skills count as demonstrated when you pass the quiz (not yet) and complete the practical tasks (not yet).
Related lessons
References and further reading
Official sources for details that can change between versions.
- What is cloud computing? (National Cyber Security Centre (UK))
- The NIST definition of cloud computing (NIST)