What is cloud computing? Someone else's computers, rented

Cloud computing is renting computing power, storage and software over the internet from a provider that owns the machines, instead of buying and running your own. The machines sit in large data centers; you reach them through a website, an app or code, use as much as you need, and pay for what you use. Most people use the cloud every day, whenever their photos back up or their email loads in a browser.

In short

• "The cloud" is real computers in real buildings, owned by someone else and shared among many customers.
• Its three main forms rent you raw machines (IaaS), a ready platform for your code (PaaS), or finished software (SaaS).
• You gain speed and flexibility, and give up some control: outages, costs at scale and lock-in are the trade-offs.

Where is the cloud, physically?

The cloud is a metaphor for a place you don't need to think about. Engineers long drew the internet as a cloud in network diagrams, a shape meaning "the network out there, details not shown", and the name stuck to the services reached through it. In reality the cloud is buildings: data centers holding tens of thousands of servers each, with their own power supplies, cooling and fiber links, run by providers in many regions of the world.

The modern business took off in 2006, when Amazon Web Services launched its storage service, S3, in March 2006, followed later that year by rentable virtual servers. A virtual server is software that behaves like a whole separate computer but shares a physical machine with others, which is what lets a provider cut one big server into many small rentals, and hand them out or take them back in seconds.

What makes something cloud computing?

The most cited definition comes from the US National Institute of Standards and Technology. Its 2011 definition calls cloud computing a model for on-demand network access to a shared pool of computing resources that can be provided and released quickly, with little effort. It lists five essential features:

• On-demand self-service: you get more computing or storage yourself, without asking a person.
• Broad network access: you reach it over the network from ordinary devices.
• Resource pooling: many customers share the same hardware, kept apart by software.
• Rapid elasticity: capacity grows and shrinks quickly, even automatically.
• Measured service: use is metered, and you are billed for it, like electricity or water.

A server rented by the year in someone else's building is hosting. It becomes cloud computing when it has those features: when you can get ten more in a minute and hand them back in an hour.

What's the difference between IaaS, PaaS and SaaS?

The three service models differ in how much the provider runs for you. A common way to picture them is a meal: cook from raw ingredients in a rented kitchen, have a meal kit delivered, or eat at a restaurant.

	Infrastructure as a service (IaaS)	Platform as a service (PaaS)	Software as a service (SaaS)
You rent	Virtual servers, storage, networks	A ready-made platform that runs your code	A finished application
You manage	The operating system, software, data and security settings	Your code and your data	Your data and settings
The provider manages	The buildings and physical hardware	Hardware, operating systems, scaling	Everything else
Everyday examples	Virtual machines rented from Amazon, Microsoft or Google	App hosting platforms and managed databases	Web email, online office suites, streaming services
Who uses it	IT teams and engineers	Software developers	Almost everyone

Most cloud services are controlled through an API, a way for programs to send requests, so a company's software can order more servers by itself when traffic rises. NIST also separates who the cloud serves: a public cloud is open to any paying customer, a private cloud is run for one organization, a community cloud is shared by a group of organizations with common needs, and a hybrid mixes two or more of these.

Why do businesses rent instead of buy?

Mostly for flexibility. Take an online shop that needs 2 servers on a normal day and 10 during one busy week a year. The numbers below are illustrative, with a server costing $0.50 an hour to rent:

Approach	Server-hours in a year	Cost at $0.50 an hour
Keep 10 running all year, enough for the peak	10 × 8,760 = 87,600	$43,800
Run 2 all year, add 8 for one week	2 × 8,760 + 8 × 168 = 18,864	$9,432

Paying only for the peak week costs about a fifth as much. A shop that owned its servers would have to buy all 10 for the busiest week and leave 8 idle the rest of the year. The cloud turns a big, up-front purchase into a running bill that follows demand. It also lets a two-person startup use the same kind of infrastructure as a large company from day one, and get servers in minutes instead of waiting weeks for hardware.

The flip side is that a running bill keeps running. A company whose demand is large and steady pays for that flexibility every month without using it.

What are the downsides of the cloud?

Outages hit many at once. When a big provider has a fault, thousands of services fail together. On 19 and 20 October 2025, a fault in automated systems in Amazon's Northern Virginia region left a key database service unreachable for nearly three hours, and knock-on effects lasted until the next afternoon, taking down apps and sites far beyond Amazon.

Lock-in. Each provider's services work in their own ways, so moving a large system to another provider can mean rewriting much of it. Fees for moving data out of a provider have long added to the cost of leaving; since 2024, under pressure from European regulators, the three largest providers waive them on request for customers who move out, but the rewriting remains.

Concentration. A few companies run most of it. According to Synergy Research Group, in the second quarter of 2026 Amazon held 28% of the worldwide cloud infrastructure market, Microsoft 20% and Google 15%, with spending of $143 billion that quarter.

Privacy and control. Your data sits on someone else's machines, often in another country, under that country's laws. Providers secure their buildings and hardware, but customers remain responsible for their own settings; many cloud data leaks come from storage that a customer left open to the public by mistake.

Still argued

Whether the cloud is cheaper is argued case by case. Supporters point to flexibility, speed and not having to hire people to run hardware. Critics point to firms that left: the software company 37signals moved seven apps off Amazon's cloud onto about $700,000 of its own servers, and by October 2024 said its yearly cloud bill had fallen from about $3.2 million to $1.3 million, according to The Register. Its savings are its own figures, and the remaining $1.3 million still went to Amazon for storage, which it planned to move as well. The usual pattern is that the cloud wins for changing or uncertain demand, and owning can win for large, steady demand.

What does the cloud mean for ordinary users?

You are almost certainly using it already: web email, photo backups, streaming, shared documents and most phone apps run on cloud servers and reach you across the internet. The practical questions are the same ones businesses ask. Who holds your data, and in which country? What happens to it if the service shuts down or you stop paying? Can you download all of it in a usable form? And is the account protected by a strong password and two-step sign-in, since in the cloud your account is the front door? Underneath, every one of these services is still ordinary hardware, working the way any computer works.

Check yourself

• Without looking back: what turns renting a server into cloud computing, by NIST's definition?
• What do you manage yourself in IaaS that you don't in SaaS?
• In the shop example, why does renting cost less, and when would owning win instead?

Sources

• The NIST Definition of Cloud Computing, SP 800-145 — Peter Mell and Timothy Grance, National Institute of Standards and Technology, 2011
• Announcing Amazon S3, Simple Storage Service — Amazon Web Services, 2006
• What is cloud computing? — Amazon Web Services
• Summary of the Amazon DynamoDB service disruption in the Northern Virginia (US-EAST-1) Region — Amazon Web Services, 2025
• Q2 cloud market passes $143 billion; highest growth rate in eight years — Synergy Research Group, 2026
• Developer pockets $2M in savings from going cloud-free — The Register, 2024