What Is an API? How APIs Work, Types & Examples

What is an API?

An API (application programming interface) is a set of rules that lets one software program request data or services from another. The API defines what you can ask for, how to ask and what you will get back, without you needing to know how the other system works inside.

What does API stand for?

API stands for Application Programming Interface:

• Application: any software with a specific function, such as a weather service, a payment system or a database.
• Programming: it is used by programs (and the programmers who write them), not directly by end users.
• Interface: a defined point of contact, a "contract" for how two things communicate.

What is an API in simple words?

Think of a restaurant. You (the app) do not walk into the kitchen (the server) to cook. You give your order to a waiter (the API), who takes it to the kitchen and brings back your food (the data). You only need to know the menu, which is the list of things the API allows you to request.

Another analogy is an electrical socket. Any device with the right plug can get power without knowing how the power plant works. An API is a standard "socket" for software.

API meaning in technology

In tech, "API" most often means a web API: an online service that apps call over the internet using HTTP. The term also covers interfaces inside a computer, such as operating system APIs that let apps use the camera or files, and programming library APIs that let developers use prewritten code.

An API example: request and response

Here is what a simple weather API call might look like. The app sends a GET request:

GET https://api.example-weather.com/v1/forecast?city=Dhaka&units=metric
Authorization: Bearer YOUR_API_KEY
Accept: application/json

The API responds with status 200 OK and a JSON body:

{
  "city": "Dhaka",
  "temperature_c": 29,
  "condition": "Partly cloudy",
  "humidity": 74,
  "updated_at": "2026-10-21T14:00:00Z"
}

The app then displays "29°C, partly cloudy" on your screen. It never needed to know how the weather service collects or stores its data, only how to ask the API.

In code, the same call takes just a few lines, for example in JavaScript:

const res = await fetch(
  "https://api.example-weather.com/v1/forecast?city=Dhaka&units=metric",
  { headers: { Authorization: "Bearer YOUR_API_KEY" } }
);
const data = await res.json();
console.log(`${data.temperature_c}°C, ${data.condition}`);

The URL and API key above are placeholders for illustration.

REST APIs explained

REST (Representational State Transfer) is the most common style for web APIs. A REST API treats everything as a resource, such as users, orders or products, each with its own URL. It uses standard HTTP methods to act on those resources.

HTTP methods (the "verbs")

Common HTTP status codes

Key REST principles

• Client-server separation: front ends and back ends evolve independently.
• Statelessness: each request contains everything needed; the server does not remember previous requests.
• Cacheable responses make repeated requests faster.
• A uniform interface with predictable URLs and methods.

Why APIs matter: benefits and the API economy

For developers and businesses, APIs:

• Save time: you do not need to build payments, maps or messaging from scratch.
• Connect systems: CRMs, ERPs, websites and apps share data automatically.
• Enable innovation: new products are built by combining existing services.
• Support every platform: one back end can serve web, mobile, smart TVs and partners.
• Protect internals: apps get only what they need, without direct database access.
• Create revenue: companies sell access to their data and services.

The API economy in numbers

Postman's 2025 State of the API Report, which surveyed more than 5,700 developers and API professionals, found that:

• 82% of organizations have adopted some level of API-first development (designing the API before the app), and 25% operate as fully API-first.
• 65% of organizations now generate revenue from their API programs.
• 89% of developers use generative AI, but only 24% design APIs with AI agents in mind.

Sources: Agile Brand Guide summary of Postman's report, Business Wire.

API development, documentation and tools

The API development lifecycle

1. Design: decide resources, endpoints, data formats and errors, often written first as an OpenAPI specification ("API-first" or "design-first").
2. Build: implement the API with a backend framework such as FastAPI, Express, Spring Boot or ASP.NET.
3. Test: check functionality, performance and security with tools like Postman and automated tests.
4. Document: publish clear reference docs and examples.
5. Deploy and secure: release behind an API gateway with authentication and rate limits.
6. Monitor and version: track usage, errors and latency, and introduce new versions (/v2) without breaking existing users.
7. Retire: deprecate old versions with plenty of notice.

What makes good API documentation?

Great API documentation is often the difference between an API people love and one they abandon. It should include:

• A quick-start guide and how to get an API key.
• Every endpoint, method, parameter and field, with examples.
• Sample requests and responses in several languages.
• Error codes and what to do about them.
• Rate limits, pagination and versioning policies.
• An interactive "try it" console.

The OpenAPI Specification (formerly Swagger) is the standard format for describing REST APIs. Tools can turn it into documentation, client code and tests automatically.

Popular API tools and platforms

An API platform usually combines several of these, such as design, testing, documentation, a gateway and analytics, so teams can manage many APIs consistently.

API vs. SDK vs. library vs. endpoint vs. webhook

These terms are often mixed up:

API vs. full stack development: full stack developers build both the front end and the back end, and the API is usually the bridge between them. The front end calls the back end's API to load and save data.

Conclusion

An API (application programming interface) is the contract that lets software talk to software. You send a request in an agreed format, and you get a predictable response back. APIs let apps reuse payments, maps, logins and AI without rebuilding them, connect businesses with partners, and now let AI agents act in the real world. Learn the basics (endpoints, methods, status codes and authentication) and you will understand how almost every modern app fits together.

Next step: pick a free public API, such as a weather or open-data API, open Postman or your browser, and make your first GET request.