Plain-English definitions of 15 common API terms. No jargon, no assumptions — just clear explanations with real examples for beginner developers.
Application Programming Interface. A set of rules that allows different software applications to communicate with each other. Think of it as a waiter in a restaurant: you (the app) tell the waiter (the API) what you want, and they bring it from the kitchen (the server).
When a weather app shows the current temperature, it's using a weather API to fetch that data from a weather service. A specific URL where an API can be accessed. Each endpoint represents a different resource or action. Think of endpoints like different departments in a store.
https://api.weather.com/v1/current — this endpoint returns current weather. https://api.weather.com/v1/forecast returns a forecast. Representational State Transfer. The most popular API design style. REST APIs use standard HTTP methods (GET, POST, PUT, DELETE) and return data — usually in JSON format. Almost every public API listed in this directory is a REST API.
GET https://api.example.com/users — retrieves a list of users (GET = read data). A unique string that identifies your application when making API requests. You get one by signing up on the API provider's website. It's like a hotel key card — it grants you access and tracks your usage.
curl -H "X-Api-Key: your_key_here" https://api.example.com/data The maximum number of API requests you can make in a given time period. If you exceed it, you get a 429 error. Free tiers typically have lower limits than paid plans.
100 requests/minute means you can call the API up to 100 times per minute. The 101st call in that minute will fail. JavaScript Object Notation. The most common format for API responses. It stores data as key-value pairs, arrays, and nested objects. It's human-readable and easy to parse in any programming language.
{"city": "Tokyo", "temp": 25, "unit": "celsius", "conditions": ["sunny", "clear"]} The type of action you want to perform on an API. GET retrieves data (safe, no side effects). POST creates new data. PUT/PATCH updates existing data. DELETE removes data. Most public APIs for reading data only use GET.
GET /api/photos — gets photos. POST /api/photos — uploads a new photo. A three-digit number in every API response that tells you whether the request succeeded or why it failed. 200 = success. 400 = bad request (your mistake). 401 = unauthorized (wrong API key). 404 = not found. 429 = rate limited. 500 = server error.
If you get 401, double-check your API key is correct and in the right header. Cross-Origin Resource Sharing. A browser security policy that controls whether JavaScript on one website can call APIs on a different domain. If an API has CORS enabled, you can call it directly from browser JavaScript. If not, you'll need a backend server.
If you're building a website and get a CORS error, you need to call the API from a Node.js server instead of directly from the browser. An authorization protocol that lets users grant your app access to their account on another service without sharing their password. More complex to implement than API keys, but required by social platforms like Twitter, Google, and GitHub.
The 'Sign in with Google' button you see on websites uses OAuth to let you log in without sharing your Google password with the site. Extra information sent along with your API request, separate from the URL. Headers commonly carry your API key (Authorization), the data format you expect (Accept: application/json), or the content type you're sending.
curl -H "Authorization: Bearer YOUR_KEY" -H "Accept: application/json" https://api.example.com Extra options added to the end of an API URL after a '?' to filter, sort, or customize the response. Multiple parameters are separated by '&'. They're the most beginner-friendly way to pass data to an API.
GET /api/weather?city=Tokyo&units=metric&lang=en — the city, units, and lang are all query parameters. A way for an API to automatically send data TO your server when something happens, instead of you constantly asking for updates. You give the API your server's URL, and it calls it when an event occurs. Like a notification system.
A payment API sends a webhook to your server URL when a customer completes a purchase, so you instantly know to fulfill the order. Software Development Kit. A pre-built library for a specific programming language that makes using an API much easier. Instead of writing raw HTTP requests, you use simple function calls. Many popular APIs have official SDKs for JavaScript, Python, and more.
Instead of writing fetch() calls, you use the SDK: const weather = await client.getWeather('Tokyo') When an API has too many results to return at once, it splits them into pages. You request one page at a time using parameters like page, limit, or offset. Always check if an API paginates when you expect more results than you're getting.
GET /api/products?page=2&limit=20 — returns items 21-40 (page 2, 20 per page).