What is an API?

In short: an API is an official entry point that lets one piece of software ask another for something, following precise rules.

You may have read “available via API” or “we plugged it into the OpenAI API.” But what is it, exactly?

What is it?

API stands for Application Programming Interface. It’s a set of rules that lets two pieces of software talk to each other, without one needing to know how the other works on the inside.

An analogy: the waiter at a restaurant

You don’t walk into the kitchen. You read the menu, which is the API’s documentation. You give your order to the waiter: that’s the request. The kitchen prepares the dish, and the waiter brings back your plate: that’s the response. You never saw the kitchen, and you don’t need to.

How does it work, in practice?

Most modern APIs work over the Web. An application sends a request to a specific address (an endpoint):

• GET to read (“give me the weather in Quebec City”);
• POST to send or create (“here’s my question for the AI”);
• PUT and DELETE to change or delete.

The response usually comes back as JSON, a text format that’s easy for machines to read, along with a code that tells you the result: 200 if all went well, 401 if you’re not authorized, 404 if what you’re looking for doesn’t exist, 429 if you’re sending too many requests.

🔑 The API key

To know who’s asking for what, and who’s paying, most services give you an API key, a long secret code sent with every request. Treat it like a password: never publish it, and don’t put it in a shared file.

You use APIs every day

• Your phone’s weather app queries a weather service’s API.
• The “Sign in with Google” button uses Google’s API.
• When an AI like ChatGPT is built into another piece of software, that software calls it through its API, and the bill is calculated in tokens (see “What is a token?”).

API or app: what’s the difference?

An app is made for you, with buttons and screens. An API is made for other software, with requests and responses. Often, the same service offers both.

Key takeaway

An API is a contract between two pieces of software: here’s what you can ask me, how to ask, and what I’ll answer.

In very simple terms: App → request → API → response → App