API Definition
Intermediate – Web/Development
What Is an API? Definition, Types, and Examples
An API (Application Programming Interface) is a set of rules that lets one software application request services or data from another. Instead of exposing its entire codebase, a program publishes an interface: a defined list of requests it will accept and the responses it will return. Developers use APIs to connect applications to an operating system, a web service, or a software library without needing to know how that system works internally.
How an API Works
Think of a waiter in a restaurant. You don’t walk into the kitchen and cook your own meal. You hand the waiter a request from a fixed menu, the kitchen prepares it, and the waiter brings back the result. An API plays the same role between two programs. The requesting application (the client) sends a call in a format the API defines. The receiving system processes it and returns a response, usually data or a confirmation.
The menu matters here. An API only accepts the requests it was designed to handle, which is exactly what makes it safe for a system to expose. The kitchen stays closed.
A weather app is the standard example. The app itself doesn’t measure temperatures anywhere. It sends a request to a weather service’s API, receives the current conditions as structured data, and displays them on your screen.
Types of APIs
APIs are usually grouped by where they operate. These three categories cover most of what developers work with day to day.
| Type | What it does | Examples |
|---|---|---|
| Operating system APIs | Let applications request OS resources such as file systems, memory, and hardware access | Windows® API, POSIX, Cocoa (macOS) |
| Web APIs | Let applications exchange data over HTTP, typically between a front end and a backend service | REST APIs, SOAP APIs, GraphQL APIs |
| Library and framework APIs | Let a program call functions from another library or framework within the same software system | Java API, React, TensorFlow |
Operating system APIs. When an application saves a file, opens a network connection, or reads keyboard input, it does so through the operating system’s API. The program never touches the hardware directly. It asks the OS, and the OS handles the rest.
Web APIs. These are APIs accessed over the HTTP protocol, and they power almost every client-server interaction on the modern web. When a front-end application retrieves, saves, or updates records in a backend database, a web API carries that request and returns the result.
Library and framework APIs. A library’s API is the set of functions and classes it exposes to your code. Import a date library and call its formatting function: that call is an API call, even though nothing left your machine. The syntax varies by programming language, but the concept stays the same.
Another common grouping is by audience. Public (open) APIs are available to any developer, partner APIs are shared with specific business partners under agreement, and private (internal) APIs stay inside a single organization.
Web API Protocols and Formats
REST (Representational State Transfer) is the dominant style for web APIs. RESTful APIs use standard HTTP methods (GET, POST, PUT, DELETE) and typically return data as JSON (JavaScript Object Notation). One clarification, because the two terms get conflated constantly: JSON is not the API. It’s the lightweight data format most REST APIs use to structure their responses, popular because it’s compact and easy for both humans and machines to parse.
SOAP (Simple Object Access Protocol) is an older, stricter protocol that exchanges XML messages. It remains common in banking, insurance, and other enterprise systems that need formal contracts and built-in error handling.
GraphQL takes a different approach. Rather than fixed endpoints, the client specifies exactly which fields it wants in a single query, which cuts down on over-fetching. Facebook open-sourced it in 2015, and it has become a popular REST alternative for complex applications.
API vs. Middleware
The two overlap, and the confusion is understandable. An API is an interface: the defined contract one program exposes so others can interact with it. Middleware is software that sits between systems and handles the connection work itself, including message queuing, translation, and authentication. Middleware often exposes its own APIs, and API gateways are themselves a category of middleware. The short version: the API is the doorway, and middleware is the corridor connecting the buildings.
Why APIs Matter for Businesses
Modern SaaS runs on API calls. Your CRM talks to your email platform, your accounting software pulls bank transactions, and your reporting dashboard aggregates data from a dozen services. Some companies expose their APIs as the product itself; Stripe and Twilio built entire businesses that way.
For teams running line-of-business software, APIs are what make integration possible without custom development for every connection. We see this constantly with the hosted applications our customers run on cloud desktops: an accounting package syncing with a tax tool, or a scheduling system feeding a reporting database, all wired together through APIs instead of manual exports.
API Definition Frequently Asked Questions
What does API stand for?
API stands for Application Programming Interface. It’s a set of defined rules that lets software applications communicate and exchange data with each other.
What is a REST API?
A REST API is a web API that follows REST (Representational State Transfer) principles, using standard HTTP methods like GET and POST and usually returning data in JSON format. It’s the most common style of web API today.
Is JSON an API?
No. JSON is a data format, not an interface. Most REST APIs use JSON to structure the data they send and receive, which is why the two terms often appear together.
What is the difference between an API and middleware?
An API is the interface a program exposes so other software can interact with it. Middleware is the software layer that manages communication between systems. Middleware frequently uses and provides APIs, but they are not the same thing.
What are examples of APIs?
The Windows API (operating system), the Google Maps and Stripe APIs (web), and the Java API (library) are common examples. Any weather app pulling live conditions from a forecast service is making web API calls behind the scenes.
Are APIs only used on the web?
No. Web APIs get the most attention, but operating system APIs and library APIs run entirely on one machine. Every time an application saves a file, it’s calling the OS’s API.
