By Quick answer: What is an API in simple terms?
- API — application programming interface.
- Analogy — a waiter between you and the kitchen.
- Examples — payments, maps, login, weather, AI models.
- REST — the most common style for web APIs.
- Security — keys, tokens, rate limits.
Key takeaways
- APIs let apps reuse capabilities instead of building everything.
- Documentation is the API's menu — read it first.
- Keep API keys secret; never ship them in app code.
- Webhooks let services notify you when something happens.
- Good APIs are consistent, versioned and well documented.
You open a food app, tap "Pay with UPI", see your order on a map, and get a message when it's out for delivery. Behind that smooth moment, a dozen separate systems are talking to each other. The thing that lets them talk is the API2.
The restaurant analogy
- You (the app) are hungry for data — say, today's weather.
- The menu (API documentation) lists what you can order and how to ask.
- The waiter (the API) takes your order to the kitchen in a format the kitchen understands.
- The kitchen (the server) prepares what you asked for.
- The waiter returns with your dish (the response) — or an apology (an error).
You never enter the kitchen. You don't need to know how it works. You just need the menu and the waiter.
APIs inside the apps you use
| What you see | API behind it |
|---|---|
| "Pay with UPI" | Payment gateway API |
| A map with your delivery rider | Maps and location APIs |
| "Sign in with Google" | Identity API |
| "Payment successful" message | A webhook from the payment gateway3 |
| An AI assistant inside an app | A model provider's API |
A request, up close
Request
GET /orders/123 — "show me order 123", with a key or token proving who's asking.
Response
200 OK plus JSON data — or 404 Not Found if it doesn't exist.
Methods
GET reads, POST creates, PUT/PATCH updates, DELETE removes.
MDN's introduction to web APIs is a good next step for the technical detail1.
Security basics
- Keep API keys and secrets on your server, never in app code
- Use HTTPS for every request
- Give each key the minimum permissions it needs
- Verify webhook signatures before trusting them
- Rate-limit your own APIs to prevent abuse
APIs are also how AI assistants use tools — see what is MCP.
Key terms, defined
API — application programming interface. Endpoint — a specific URL an API responds to. REST — a common web API style. JSON — a text format for data. Webhook — a callback sent when an event happens. API key — a secret identifying the caller.
Frequently asked questions
What is the full form of API?
Application Programming Interface.
What is a REST API?
A common style of web API that uses standard HTTP methods — GET, POST, PUT, DELETE — on URLs representing resources, usually exchanging JSON.
What is an API key?
A secret identifier that tells an API who is calling. Keep keys on your server, not in mobile or web app code.
What is the difference between an API and a webhook?
With an API, your app asks for information. With a webhook, the other service calls your app automatically when something happens, such as a payment succeeding.
Do non-developers need to understand APIs?
A basic understanding helps founders, PMs and designers estimate effort, evaluate tools and talk with engineers.
Is UPI an API?
UPI is a payment system run by NPCI; payment apps and gateways connect to it through interfaces built on its specifications, which in practice work like APIs.
Sources
- MDN Web Docs — Introduction to web APIs
- AWS — What is an API?
- Razorpay Docs — Webhooks
- Checked October 23, 2026. Figures that change — fees, salaries, prices — are reviewed every six months; if something is out of date, tell us.
Where to go from here
- What is MCP? — how AI uses tools
- Best backend for mobile apps — where your API lives
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