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Intermediate

What is an API

Full name: Application Programming Interface

Also known as: application programming interface, API integration, API connection

Definition

An API (application programming interface) is the agreed way one program asks another for data or an action and gets an answer, so your tools can work together.

Why it matters

APIs are how your tools talk to each other: the website to the payment provider, the shop to the invoicing platform, the booking form to the CRM.

Common mistakes

Sending an API key by email or WhatsApp, or leaving it in public code, which hands over access to your account.

Related terms

Worked exampleIllustration

An API call: a paid online workshop issues its invoice

The request

  • Address, the endpointapi.example.com/invoices
  • Action: create something newPOST
  • Key: proves who is asking, kept secret••••••
  • Details: customer, seat, amountJSON

The answer

  • Status: created201
  • The new invoice number[number]

When it fails

  • Missing or wrong key401
  • Wrong address404

Steps

  1. A customer pays for a seat.

  2. The workshop's system sends a POST request to the invoicing platform's API address, with its API key and the order details.

  3. The platform checks the key, creates the invoice and answers with status 201, created, and the invoice number.

  4. The system saves the invoice number next to the order and emails the invoice to the customer.

  5. If the key is replaced or the platform changes its format, invoices stop, so an alert on failed calls tells the owner the same day.

The request carried an address, an action, a key and the details, and the answer carried a status and a result: 201 when something new was created, 200 when data was only read.

Illustration

Pick a request from the menu

The request

Action
POST
Address
api.example.com/invoices
Key
••••••
Details
JSON

The answer

Status
201
Comes back
The new invoice number [number]

POST asks the platform to create something new. Status 201 means it was created, and the new invoice number comes back to be saved next to the order.

An API is a menu one program offers another. Each request names an action, an address and a key, and the answer comes back with a status code.

You ask when you need it, or you are told

Asking every 30 minutes: 21 questions today, 20 of them answered "nothing new", and the payment was noticed 23 minutes late. The webhook sent one message, at the moment of payment.Payment 11:07⁨API: your system asks⁩⁨+23 min⁩⁨Webhook: the provider tells you⁩200 OK08:0013:0018:00

Asking every 30 minutes: 21 questions today, 20 of them answered "nothing new", and the payment was noticed 23 minutes late. The webhook sent one message, at the moment of payment.

If your server is down when the webhook arrives, most providers try again later, so check that the same event is not handled twice.

An API call is the top lane: your system asks when it needs something. A webhook is the bottom lane: the other system tells you when something happens. Most integrations use both.

Definition

An API is a set of features and rules inside a software program that lets other software work with it, as opposed to a screen made for a person. A web API call has four parts. The address is the endpoint, one specific URL inside the API. The action says what to do, such as GET to read or POST to create. The key proves who is asking. The answer usually carries data in JSON plus a status code, such as 200 for success or 404 for not found. Many web APIs follow a common style called REST. Think of a restaurant: the menu is the documentation, you order through the waiter, and you never enter the kitchen.

Why it matters

The invoicing or payment system you already use probably has one, and Tranzila, Morning (Green Invoice), iCount and Cardcom all publish public API documentation. An API key is a password for software, so it belongs in a secret setting, never in an email, a WhatsApp message or public code. Many APIs cap how many calls you can make, and charge per call or per month above a free tier. Before you pay anyone to build an integration, ask three questions: is there a public API, is there a ready made plugin or connector, and what does each call cost?

Directly related: Webhook, Back-end, Client (computing).

Improvement tips

  • Keep API keys in a secret setting or a password manager, give each integration its own key, and replace a key the moment it leaks.
  • Set an alert on failed calls, so a broken integration is noticed the same day and not at the end of the month.
  • Before paying for a custom integration, check whether a ready made plugin or connector already does the job.
  • Read the provider's limits and prices per call before your daily work depends on its API.
  • If you publish your own API, document it and version it, for example /v1/ and /v2/, so updates do not break the people who use it.

Common mistakes

  • Giving an integration more permissions than it needs, such as full account access for a job that only reads orders.
  • Paying for a custom integration when a ready made connector already existed.
  • Ignoring the provider's notices that an old version is being retired, until the integration stops.
API vs Webhook

Related terms

TechnologyIntermediate

Webhook

A webhook is an automatic message one system sends to an address you gave it the moment something happens, like a payment. You stop asking, it tells you.

TechnologyIntermediate

Back-end

Back-end is the part of a website or app that users never see: it saves orders, checks stock, charges cards and keeps accounts, running on a server.

TechnologyBeginner

Client (computing)

A client, in computing, is the program, such as a browser or an app, that sends a request to a server and shows you the answer. It is not your paying customer.

TechnologyIntermediate

Authentication

Authentication is the check that a user is who they claim to be, using something they know, something they have or something they are.

TechnologyBeginner

Database

A database is an organised collection of information, usually tables of rows and columns, that software can search, update and link, like customers to orders.

Quick check

Your developer asks for your invoicing platform's API key. How should you treat it?

Choose an answer

Frequently asked questions

Do I need to understand APIs before I start my business?
Not the code, but the idea: an API lets one piece of software ask another for data or an action. It tells you that your site, payments, invoicing and CRM can be connected instead of copied by hand.
When does an API first become relevant for a new business?
When you want two tools to share data: payments on the site, leads into the CRM, an invoice for every order. Often the connection already exists as a plugin that uses the API behind the scenes.
How do API integrations affect my launch timeline?
Connecting to an existing service is much faster than building the same feature yourself, such as shipping labels from a courier or invoices from an invoicing platform. How fast depends on the provider's documentation and on whether a ready made connector exists.
Should I build my own API or use existing ones?
Use existing ones first. Build your own only when partners or your own mobile app need to reach your data, and then document it and version it so your changes do not break them.
Why does API reliability matter for a business already running?
If a provider's API is slow or down, the part of your site that depends on it slows down or stops, such as the checkout when the payment API fails. Monitoring the connection tells you when to call the provider or switch.
What goes wrong when a business ignores API updates?
Providers retire old versions after a notice period. If nobody acts on the notice, the integration stops on the day the old version closes, and the checkout, the invoices or the lead syncing break.
How do I fix a broken API connection without stopping day to day work?
A developer can make the system fail gracefully: put the messages in a queue and send them later, when the service is back, and test the fix on a staging copy before release. Meanwhile the rest of the site keeps working.
How do I know if my website is using too many APIs?
If pages wait for several outside services before they appear, they load slowly. A developer can see in the browser's network panel which calls take the longest, then drop some or load them in the background.
What does API actually mean in plain words?
A menu one program offers another. Your system orders from the menu, the provider's system prepares the answer, and neither one enters the other's kitchen.
Are API integrations risky for my business security?
An integration is as safe as its key handling and the permissions it is given. An API key is a password to your account, so keep it secret, give each integration only the access it needs, and cancel keys you no longer use.
Do I need a programmer to connect APIs to my website?
Not always. Automation tools such as Zapier or Make, and plugins for your site builder, connect common tools without code. For custom data, many systems or payments, a developer makes the connection reliable and secure.
Is an API the same thing as a webhook?
No, they work in opposite directions. With an API your system asks when it needs something. With a webhook the other system tells you the moment something happens. Many integrations use both: a webhook says an order was paid, and an API call then issues the invoice.

Sources: MDN Web Docs, Glossary: API (developer.mozilla.org/en-US/docs/Glossary/API), read 23 September 2026, IETF, RFC 9110, HTTP Semantics, 2022, sections 9 and 15

Last reviewed: 2026-09-23

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