You use APIs every day without realising it: whenever a site displays a map, accepts a payment, or offers an AI response, software programs have talked to each other behind the scenes. Here is the concept, without the jargon.

The restaurant analogy

Picture a restaurant:

• You (the customer) do not walk into the kitchen.
• You go through the waiter: you place your order, they pass it on, they bring back the dish.
• The kitchen prepares it, without you knowing how.

An API (Application Programming Interface) is that waiter: a go-between that takes a request, passes it to the right software, and brings back the response. You do not need to know how the kitchen works.

What it actually does

Rather than rebuilding everything, a site calls the API of a specialist:

• Maps → Google Maps / OpenStreetMap API.
• Payment → Stripe / PayPal API.
• Weather, exchange rates, transport → APIs from data providers.
• AI → APIs from Claude, OpenAI… (the site sends a text, receives a response).

This is the principle of the reusable building block: you assemble services instead of reinventing the wheel.

How it works (in brief)

1. A program sends a request to an address (an API URL), optionally with parameters.
2. The API processes it and returns a response, most often in JSON format (structured, lightweight text).
3. The program uses that data (displays it, saves it…).

All of it in a fraction of a second, with no web page: it is a conversation between machines.

The API key

Many APIs require an API key: a secret code that identifies the caller. It makes it possible to know who is calling, to count usage and to bill or limit it. Golden rule: an API key must never be put in public code or on a visible page — it is a secret.

Why it is central today

APIs are the glue of the modern web: they connect static sites, Node.js applications, databases and AI. The famous Jamstack approach rests entirely on them: a fast front end that fetches its data through APIs.

Understanding APIs means understanding how everything connects.