Application Programming Interface definition
An API (Application Programming Interface) is a defined contract that lets one piece of software request data or actions from another without knowing how it works inside. Web APIs typically accept HTTP requests and return JSON, so a mobile app, partner system or AI agent can use a service such as payments, maps or a database safely and consistently.
How does an API work?
An API defines the requests a client may make, the data each request needs and the responses it will get back. In a typical web API, a client sends an HTTP request such as GET /orders/123 with an authentication token. The server validates the token, runs its own logic, reads a database and returns a response, usually JSON with a status code such as 200 for success or 404 when the order does not exist.
The client never sees the server's code or database. That separation is the point: the team behind the API can rewrite its internals, change databases or add servers, and every client keeps working as long as the contract stays the same. Our HTTP status codes reference lists the response codes APIs use most often.
Types of APIs
APIs differ in style, transport and who is allowed to call them. Public APIs are open to any registered developer, partner APIs to selected companies and private APIs only to a company's own apps and services. The most common technical styles are:
- REST APIs: resources addressed by URLs and changed with HTTP methods, the most common style for public web APIs (see REST API)
- GraphQL: a single endpoint where clients ask for exactly the fields they need, popular for complex front ends
- gRPC: binary, contract-first calls over HTTP/2, common between internal microservices
- Webhooks: the reverse direction, where a service calls your URL when an event happens
- Library and operating system APIs: functions exposed inside code, such as the browser Fetch API or Android's camera API
Examples of APIs in everyday products
When a food delivery app shows a map, it calls a maps API. When it charges a card, it calls a payment API such as Stripe or Razorpay, which returns success or failure without the app ever storing the card number. When the restaurant's tablet receives the order, it is listening to the platform's order API. A single screen can depend on a dozen APIs from different companies.
AI products work the same way. A chatbot sends a prompt to a model provider's API and receives generated text, and an AI agent uses function calling to decide which of your own APIs to call next, turning existing endpoints into tools the model can use.
API security and design best practices
Every API is an entry point, so authenticate every call with OAuth 2.0 or signed tokens, authorize access per resource, validate input, apply rate limits and log access. Version the API so breaking changes do not surprise clients, document it with an OpenAPI specification and return consistent error formats. Many real API breaches come from broken object-level authorization, where a user fetches another user's record simply by changing an ID in the URL.
Nexzem designs and builds APIs for web, mobile and partner integrations, starting from an OpenAPI contract that front-end and back-end teams agree before coding, so both sides can build in parallel against mocks and integration surprises surface early rather than in the week before launch.