APIs power the modern web — but the term is often misunderstood. This guide explains what an API is, how REST and webhook APIs work, and why businesses need them, using plain-English examples.
In this guide
An API (Application Programming Interface) is like a restaurant menu and waiter. You (the customer/your app) tell the waiter (the API) what you want. The waiter takes your order to the kitchen (the other system) and brings back the result. You do not need to know how the kitchen works — you just need to know what you can order (the API documentation) and how to place the order (the API format).
When your website's booking form saves an appointment, it calls an API. When you see a Google Map embedded on a page, it uses the Google Maps API. When your e-commerce store processes a payment, it calls the Stripe API. When your app sends a Slack notification, it calls the Slack API. APIs let software systems talk to each other — enabling integrations that would otherwise require building every feature from scratch.
A REST API uses HTTP — the same protocol as web browsers. It has endpoints (URLs), methods (GET to read data, POST to create, PUT/PATCH to update, DELETE to remove), and returns data usually in JSON format. Example: GET https://api.yourapp.com/users/123 returns {"id":123,"name":"Alice","email":"alice@example.com"}. REST APIs are stateless — each request contains all information needed, with no memory of previous requests.
A regular API call: your system asks another system for data. A webhook: the other system proactively notifies your system when something happens. Example: instead of polling Stripe every minute to check if a payment succeeded, Stripe calls your webhook URL the moment a payment is completed. Webhooks are more efficient and deliver data in real time. The tradeoff: you need to expose a public URL for the other system to call.
Most APIs require authentication — you need to prove you are authorized to use them. The most common method is an API key: a string like sk_live_abc123 that you include in every request as a header or query parameter. Treat API keys like passwords: store them in environment variables, never commit them to git, rotate them if they may have been exposed. More secure APIs use OAuth 2.0 for user-level access (see our OAuth guide).
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