What is an API? A plain-English guide for beginners

"API" appears in almost every developer job description, and in most interviews someone will ask you to explain it. This guide explains what an API is in plain English, with one everyday example and one small piece of code, so you can describe it with confidence even if you have never built one.

The restaurant example

Imagine you are at a restaurant. You do not walk into the kitchen and cook. Instead:

• You read the menu, which lists what you can order.
• You tell the waiter what you want.
• The waiter takes your order to the kitchen and brings back your food.

An API (Application Programming Interface) is the waiter and the menu together. It is an agreed way for one program to ask another program for something, without needing to know how the other program works inside.

When a weather app on your phone shows today's temperature, it did not measure it. It sent a request to a weather service's API and displayed the response.

What a web API request looks like

Most APIs you will meet at work are web APIs: you send a request over the internet to a URL, and you get a response back, usually as JSON (a simple text format for data).

A request has four main parts:

The four methods you will see most often:

• GET reads data ("show me the menu").
• POST creates something new ("place an order").
• PUT or PATCH updates something ("change my order").
• DELETE removes something ("cancel my order").

A tiny example

Here is how you might ask a hypothetical books API for one book, using Python's popular requests library:

import requests

response = requests.get("https://api.example.com/books/42")

print(response.status_code)  # a number such as 200 or 404
print(response.json())       # the data, turned into a Python dictionary

The server would reply with JSON shaped something like this:

{
  "id": 42,
  "title": "Clean Code",
  "author": "Robert C. Martin",
  "available": true
}

Status codes: the waiter's reply

Every response comes with a status code, a three-digit number that tells you how things went. You only need a few to start:

• 200 OK: it worked, here is your data.
• 201 Created: your new item was created.
• 400 Bad Request: something is wrong with what you sent.
• 401 Unauthorized: you need to log in or send a valid key.
• 404 Not Found: that item does not exist.
• 500 Internal Server Error: the server had a problem; it is not your fault.

A good habit: when something fails, check the status code before anything else. It usually tells you whether to fix your request or wait for the server.

How to explain an API in an interview

Keep it short and concrete:

"An API is a defined way for two programs to talk. One side sends a request to an endpoint, with a method like GET or POST, and the other side sends back a response, usually JSON, with a status code. For example, a weather app calls a weather API to get today's forecast."

Then, if you can, mention something you built that called an API. Even a small project that fetches data and shows it on a page counts. Our list of portfolio projects for freshers has ideas.

Where to go next

Once calling an API feels natural, read our notes on designing REST APIs for AI features to see how real teams structure them. If you are unsure about AI words like "model" and "token", start with AI terms explained for beginners.