The client sends a request
A web page, mobile app or another server sends an HTTP request to an endpoint, for example GET https://api.example.com/v1/products/42.
What Is an API in Web Development? A Plain-English Guide
An API (application programming interface) is a set of rules that lets one piece of software request data or actions from another. In web development, an API usually means a web API: the front end of a website or mobile app sends an HTTP request to a server endpoint such as /api/orders, and the server replies with data, typically in JSON. APIs are how your app shows a map, takes a card payment, logs users in with Google or loads products from a database. This guide explains how APIs work step by step, the main types and architectures (REST, GraphQL, gRPC, webhooks), real examples from everyday web apps, how to secure and document an API, and best practices for building one.
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- An API is a contract: it defines what you can ask for, how to ask, and what you get back.
- Web APIs use HTTP methods (GET, POST, PUT, PATCH, DELETE), endpoints (URLs), headers, a body and status codes.
- REST is the most common style; GraphQL, gRPC and webhooks suit specific needs.
- APIs let teams reuse functionality, connect third-party services and power web and mobile apps from one back end.
- Secure APIs with HTTPS, authentication (API keys, OAuth 2.0), authorisation checks, rate limits and input validation.
What Is an API?
What Is an API?
How an API Works, Step by Step
The API checks who is asking
It reads the API key or access token in the request headers and confirms the caller is allowed to do this.
The server does the work
Business logic runs: it might read a database, call another service or start a payment.
The API sends a response
The server returns a status code (such as 200 OK or 404 Not Found) and a body, usually JSON, for example the product's name, price and stock level.
The client uses the data
The front end displays the product, updates the cart or shows an error message.
Parts of an API Request and Response
| Part | What it is | Example |
|---|---|---|
| Endpoint | The URL of the resource | /v1/products/42 |
| Method | What you want to do | GET to read, POST to create, PUT or PATCH to update, DELETE to remove |
| Headers | Extra information about the request | Authorization: Bearer token; Content-Type: application/json |
| Query parameters | Filters or options in the URL | ?category=shoes&page=2 |
| Body | Data sent with POST, PUT or PATCH | A JSON object with the new order details |
| Status code | Result of the request | 200 OK, 201 Created, 400 Bad Request, 401 Unauthorized, 404 Not Found, 500 Server Error |
| Response body | Data returned | A JSON object with the product name, price and stock |
API vs Endpoint vs Web Service
| Term | Meaning |
|---|---|
| API | The whole interface: all the operations, rules, data formats and authentication a service offers |
| Endpoint | One specific URL and method in that API, such as POST /orders |
| Web service | An API reached over a network, usually HTTP; all web services are APIs, but not all APIs are web services (for example, browser and operating system APIs) |
| SDK | A ready-made code library that calls an API for you in a given language |
Types of APIs by Who Can Use Them
Public (open) APIs
Available to any developer, often with an API key and usage limits. Examples: weather data, maps and currency exchange APIs.
Partner APIs
Shared with selected business partners under an agreement, such as a travel supplier giving resellers access to its booking inventory.
Private (internal) APIs
Used only inside a company, for example the API your own website and mobile app share to read orders and customer accounts.
Composite APIs
Combine several calls into one, such as creating a customer, an order and a payment in a single request, which reduces round trips for mobile apps.
API Architectures Compared
| Architecture | How it works | Best for | Example |
|---|---|---|---|
| REST | Resources at URLs, standard HTTP methods, usually JSON | Most web and mobile apps, public APIs | GET /users/7 returns a user |
| GraphQL | One endpoint; the client asks for exactly the fields it needs | Complex front ends with many related data types | One query returns a user, their orders and reviews |
| gRPC | Binary protocol with strict contracts (Protocol Buffers) | Fast communication between back-end services | Microservices calling each other thousands of times a second |
| SOAP | XML messages with a formal contract (WSDL) | Older enterprise, banking and government systems | Legacy payment or insurance integrations |
| Webhooks | The server calls your URL when an event happens | Notifications and syncing | A payment provider tells your app a payment succeeded |
| WebSockets | A two-way connection that stays open | Real-time chat, live dashboards, games | Live order tracking or stock prices |
Real Examples of APIs in Web Apps
| Feature on a website or app | API behind it |
|---|---|
| Sign in with Google or Apple | OAuth 2.0 and OpenID Connect identity APIs |
| Card and UPI payments | Payment gateway APIs such as Stripe, Razorpay or PayPal |
| Maps and delivery tracking | Google Maps Platform or Mapbox APIs |
| Order confirmation SMS and email | Twilio, SendGrid or AWS SES APIs |
| Product search | Algolia or Elasticsearch APIs |
| AI chat or content features | Large language model APIs from providers such as OpenAI, Anthropic or Google |
| Shipping rates and labels | Courier APIs such as Shiprocket, Delhivery, FedEx or UPS |
APIs by Industry
Ecommerce
Product, cart, payment, inventory and shipping APIs connect storefronts, marketplaces and warehouses.
Healthcare
HL7 FHIR APIs exchange patient records, appointments and lab results between hospital systems and patient apps.
Financial services
Open banking and payment APIs share account data with consent and move money securely.
Logistics
Tracking, routing and carrier APIs give customers live delivery updates and automate dispatch.
Why APIs Matter in Modern Web Development
One back end, many front ends
The same API serves the website, mobile apps and partner tools.
Faster development
Teams reuse ready-made services for payments, maps or messaging instead of building them.
Independent teams
Front-end and back-end teams can work in parallel against an agreed contract.
Scalability
API-based services can be scaled and updated separately.
New revenue
Businesses can offer data or services to partners through paid APIs.
How to Build an API for a Web Application
Define what the API must do
List the resources (users, products, orders) and the actions each consumer needs.
Design the contract first
Write an OpenAPI specification with endpoints, request and response formats, errors and authentication, and review it with front-end and mobile teams.
Choose the stack
Popular choices are Node.js with Express or NestJS, Python with FastAPI or Django REST Framework, Java with Spring Boot, PHP with Laravel and C# with ASP.NET Core.
Build and test
Implement endpoints, validation and error handling, and add unit, integration and contract tests.
Secure and deploy
Add authentication, rate limits and HTTPS, then deploy behind an API gateway with logging and monitoring.
Document and version
Publish interactive documentation and version the API so changes never break existing apps.
API Security Basics
| Practice | Why it matters |
|---|---|
| HTTPS everywhere | Encrypts data in transit so tokens and personal data cannot be read |
| Authentication (API keys, OAuth 2.0, JWT) | Confirms who is calling the API |
| Authorisation on every request | Stops users reading or changing other users' data, the most common API flaw in the OWASP API Security Top 10 |
| Input validation | Blocks injection attacks and malformed data |
| Rate limiting and quotas | Prevents abuse, scraping and denial-of-service |
| Minimal responses | Returns only the fields the client needs, reducing data exposure |
| Logging and monitoring | Detects unusual traffic and helps investigate incidents |
API Documentation and Testing Tools
| Tool | Used for |
|---|---|
| OpenAPI (Swagger) | Describing REST APIs and generating interactive docs and SDKs |
| Postman | Sending test requests, sharing collections and automated tests |
| Insomnia | Lightweight API testing and debugging |
| GraphiQL and Apollo Studio | Exploring and testing GraphQL APIs |
| k6 and JMeter | Load and performance testing |
API Best Practices
Use clear, consistent naming
Plural nouns for resources (/orders, /orders/123) and HTTP methods for actions.
Return meaningful status codes and error messages
For example 422 with a message that says which field is invalid.
Paginate large lists
Return results in pages with limits and cursors.
Version your API
/v1/ in the path or a version header, with notice before retiring old versions.
Cache where you can
Use ETags and cache headers for data that changes rarely.
Keep it idempotent
Repeating the same PUT or DELETE should not cause extra side effects; use idempotency keys for payments.
Related Reading
Related Reading
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Frequently Asked Questions
What is an API in simple terms?
An API is a set of rules that lets one program ask another for data or actions, like ordering from a menu without going into the kitchen.
What is an API in web development?
It is usually a web API: a set of HTTP endpoints that a website or app calls to get or send data, typically as JSON.
What is an example of an API?
When a shopping site shows a map of your delivery, it calls a maps API. When you pay by card, it calls a payment gateway API.
What are the main types of APIs?
By access: public, partner, private and composite. By architecture: REST, GraphQL, gRPC, SOAP, webhooks and WebSockets.
What is the difference between an API and a REST API?
API is the general idea. A REST API is a specific style of web API that uses URLs for resources and standard HTTP methods.
What is the difference between REST and GraphQL?
REST has many endpoints that return fixed data shapes. GraphQL has one endpoint where the client asks for exactly the fields it needs.
What is an API endpoint?
An endpoint is one specific URL and method in an API, such as GET /products/42.
What does JSON have to do with APIs?
JSON is the most common data format web APIs use to send and receive data because it is light and easy for browsers and servers to read.
Are APIs secure?
They are secure when built properly, with HTTPS, authentication, per-request authorisation, input validation and rate limits.
What is an API key?
An API key is a unique code that identifies the app calling an API. It is often combined with OAuth tokens for user-level access.
Do front-end developers need to know APIs?
Yes. Front-end developers call APIs to load and save data, so they need to understand requests, responses, status codes and authentication.
How much does it cost to build an API?
A simple API for one app can cost $5,000–$15,000, an integration API $15,000–$40,000 and an enterprise API platform $60,000 or more, depending on scope and security needs.