What is an API (Application Programming Interface)?
An API, short for Application Programming Interface, is a set of rules that lets one piece of software ask another for data or an action. It defines what you can request, how to ask for it, and what you get back. When a weather app shows you tomorrow's forecast, it does not store the weather itself. It calls a weather service through an API and displays the answer.
You use APIs all day without noticing. Logging in with Google, paying with a card online, seeing a map embedded in a page: each one is an API quietly passing a request from one system to another.
In plain words
Think of an API as a waiter in a restaurant. You do not walk into the kitchen and cook your own meal. You read the menu (the list of things you can order), tell the waiter what you want, and the waiter brings it back. The kitchen stays hidden, you only deal with the menu and the waiter. An API is that menu and that waiter: a clear list of what you can ask for, and a messenger that carries the request and the reply.
How APIs work
Most APIs follow a simple pattern: a request goes out, a response comes back.
- The request says what you want. It goes to a specific address called an endpoint, for example
api.weather.com/forecast. You can attach details, such as the city and the date. - The service receives the request, does the work (looks up the data, saves a record, runs a calculation), and sends an answer.
- The response carries the result, usually as structured data in a format called JSON that both humans and programs can read.
A few terms you will keep meeting:
- Endpoint is the specific address you call for one kind of action.
- Request and response are the question and the answer.
- API key is a password that proves who is calling, so the service can recognise you and count your usage.
The point is that you never need to know how the other system works inside. You only need to know what to ask for and what you will get back.
Why APIs matter
APIs are the reason modern software gets built quickly instead of from scratch every time.
- You reuse instead of rebuild. Need payments? Call Stripe. Need maps? Call Google Maps. You skip months of work by asking an existing service.
- Systems connect. Your e-shop talks to your warehouse, your invoicing, and your shipping provider through their APIs, so an order flows without anyone retyping it.
- AI tools depend on them. When a chatbot answers from your documents or an agent books a meeting, it is calling APIs in the background. Tools like ChatGPT and Claude are themselves available through APIs, which is how companies build AI into their own products.
For a business this means faster delivery and fewer custom integrations. The better an API is documented and designed, the faster other teams can build on top of it.
Common pitfalls
- No live data without a connection. An API only returns what the service behind it actually has. If the data is stale or the service is down, you get stale answers or nothing.
- Rate limits. Most APIs cap how many requests you can make in a given time. Hit the limit and your calls start failing, often at the worst moment.
- Breaking changes. When a provider changes its API, code that relied on the old version can break. This is why serious APIs use versions and clear documentation.
- Leaked API keys. A key is a password. If it ends up in public code, someone can run up your bill or access your data. Keep keys secret and rotate them when in doubt.
Related articles:
- SDK: What is it and how can it help my business? - The toolkit developers use to build on top of your product.
- Understanding the distinction: SDK vs. API in software development - Related ideas with different roles, explained side by side.
- Improve API adoption with the OpenAPI Specification - The common standard for documenting and consuming REST APIs.
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