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What is an API? Definition, how it works and examples
An API, or application programming interface, is a mechanism that lets two pieces of software communicate. It defines the rules by which one application can request, send or modify information in another, without needing to know how that other application works inside.
In one sentence
An API is an intermediary that lets two applications exchange data or trigger actions.
Key points
- An API lets different systems communicate without knowing each other.
- It states precisely which data and which actions are reachable from outside.
- It can be public, restricted to partners, or strictly internal.
- On the web it usually runs over HTTP and exchanges data as JSON.
Term at a glance
- API
- Application Programming Interface
- French term
- Interface de programmation applicative
- Domain
- Web development
- Category
- Software architecture
- Level
- Beginner to intermediate
What exactly does "API" mean?
The word interface means a point of contact: where two systems meet. An API is therefore the contact point a piece of software deliberately exposes so other programs can plug into it. It is not the software itself, but the contract describing what may be asked of it.
That contract states three things: the available operations, the shape of the data expected, and the shape of the replies returned. As long as the contract holds, the provider can rewrite its software entirely without users noticing. That decoupling is what makes an API valuable.
The problem it solves is an old one: without an API, making two systems talk requires each to know the other's internals. The smallest change on one side breaks the other. An API replaces that brittle dependency with a stable, documented boundary.
How does an API work?
- 01
An application makes a request
The calling program sends a request to a specific address on the API, stating the operation it wants and the information required.
- 02
The API checks authorisation
It verifies who is calling and what they may do: a key, a token or a certificate determines their access.
- 03
The request is processed
The system carries out the operation: a database read, a computation, a write, or triggering a process.
- 04
A reply is returned
The API returns the result in a structured format, with a code saying whether the operation succeeded or why it failed.
A concrete API example
A customer books an appointment on your website. On confirmation, the site queries your calendar software's API to check availability, then hands it the appointment to record. The calendar confirms, and the site shows the customer their booking. The two pieces of software come from different vendors and were never designed together: the API is what makes the exchange possible.
What is an API used for?
Opening a service to partners
Letting a distributor or a customer check your stock or place an order from their own system.
Centralising scattered data
Pulling information from several sources into one dashboard, with no manual file exports.
Reusing an existing service
Adding payment, mapping, email delivery or translation without rebuilding those pieces.
Advantages and limits of an API
- Systems evolve separately without breaking each other
- Exchanges become automatic, traceable and repeatable
- Access is controlled finely, operation by operation
- One interface serves several client applications
- It adds a dependency: if the provider goes down, your feature goes down
- It must be versioned, or a change breaks every caller
- It widens the exposed surface and demands serious authentication
- It introduces network latency that direct access would not have
What is the difference between an API and a webhook?
| API | Webhook | |
|---|---|---|
| Trigger | The calling application asks when it needs to | An event fires the send automatically |
| Direction | You query the service | The service notifies you |
| Typical case | Requesting today's order list | Being told the moment an order is placed |
| Freshness | Depends how often you ask | Immediate |
Why does an API matter to a business?
An API turns isolated software into a system that works together. In practice: data entered once, re-keying that disappears, transcription errors avoided, and the ability to swap one tool without rebuilding everything. It also makes a company integrable: a partner or customer who can plug into your system becomes much harder to lose.
Frequently asked
Is an API a piece of software?
No. It is an interface exposed by software. The software holds the logic and the data; the API only describes what can be asked of it from outside, and in what form.
What is the difference between an API and a database?
A database stores the information. An API controls who can reach it and how. Exposing a database directly would let anyone read and change everything; an API opens only the operations you intended.
What is a REST API?
It is a widespread style of web API, built on the verbs of the HTTP protocol and on addresses that name resources. Its popularity comes from its simplicity: any tool that can make a web request can use it.
Is an API secure?
It is if it was designed to be. Authentication, rate limiting, validation of incoming data and logging are all essential. A poorly protected API is a direct door into your data.
Can you use an API without knowing how to program?
Partly, yes. Automation tools can connect services by API without writing code. Once the logic gets complicated or volumes grow, development becomes the better route.
Related terms
Sources and references
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