Application Programming Interface

An application programming interface, or API, is a defined set of rules and tools that lets one piece of software communicate with another, specifying exactly what requests one program can make of another, what information must be included in that request, and what kind of response it can reasonably expect to receive back in return. An API acts as a kind of contract between two pieces of software, letting a developer directly use another program’s functionality or data without needing to know anything at all about how that other program is actually built internally, much like a restaurant menu lets a customer order a dish without knowing exactly which steps the kitchen follows to prepare it.

Web APIs and How They Work

The most common type of API encountered on the modern internet is the web API, which lets a program on one computer request data or services from a program running on a remote server over the internet. A weather app on a phone, for example, typically does not calculate forecasts itself; instead, it sends a request through an API to a remote weather service’s server, which processes that request and sends back the relevant forecast data, which the app then displays to the user in its own particular visual format.

Many modern web APIs follow a design style called REST, which structures requests around standard web addresses representing specific resources, such as a particular user’s profile page or a specific product listed in a store, combined with standard web methods that specify the type of action being requested, such as retrieving, creating, updating, or deleting that resource. Data is commonly exchanged in a lightweight, widely supported format called JSON, which represents information as a straightforward structure of named values that is easy for both humans to read and software written in virtually any modern programming language to parse and generate reliably and consistently.

Why APIs Matter

APIs let software systems that were built by entirely different teams, companies, or even at entirely different points in time work together without each needing to understand the other’s full internal complexity. A single mobile app commonly relies on dozens of separate APIs behind the scenes, handling tasks including processing payments, verifying a user’s identity, sending push notifications, translating text, and displaying an embedded map, each provided by a different specialized company through its own dedicated API rather than the app’s own developers needing to build every one of those capabilities completely from scratch themselves.

Access to many APIs is controlled through API keys, unique codes that identify who is making a request and allow the provider to track usage, enforce rate limits that cap how many requests a given user can make within a certain time period, and bill appropriately for higher-volume usage when the API is offered as a commercial, paid product. Well-designed APIs are typically documented thoroughly, with clear explanations of every available request type, the exact data format expected, and realistic example requests and responses, since a poorly documented API, however technically capable and powerful it might actually be, is often frustrating and genuinely difficult for other developers to use effectively and confidently in their own projects.

From Internal Libraries to a Global Ecosystem

The term API originally referred to a much broader concept, describing any defined interface that let one piece of software code call functions provided by another piece of code, including interfaces entirely internal to a single program or operating system, long before the modern web existed. The rise of the web and, especially, smartphone apps in the 2000s and 2010s shifted the term’s everyday meaning toward web APIs specifically, as companies increasingly exposed parts of their services to outside developers over the internet rather than keeping every capability locked away purely for internal use.

This shift gave rise to a large, interconnected ecosystem sometimes called the API economy, in which businesses generate meaningful revenue by offering useful capabilities, such as payment processing, mapping, messaging, or artificial intelligence services, to other companies through well-documented, broadly accessible APIs rather than exclusively through their own direct customer-facing products. A single modern app or website frequently combines capabilities from numerous outside companies in this way, each reached through its own API, letting smaller teams build sophisticated products by assembling specialized external services rather than needing to build every single underlying capability themselves from the ground up.

Frequently Asked Questions

What is the difference between an API and a website? A website is designed primarily for a human being to view and interact with directly through a browser, while an API is designed for one piece of software to communicate directly with another, typically exchanging structured data rather than a visually formatted page meant for direct human reading.

What does REST mean in the context of an API? REST describes a common architectural style for building web APIs that organizes requests around resources identified by standard web addresses, combined with standard web methods indicating the type of action being requested, such as retrieving, creating, or updating a given piece of data.

Why do many APIs require an API key? An API key identifies who is making each individual request, which lets the provider track usage, enforce limits on how many requests a given user can make within a given period, and, for commercial APIs, bill appropriately for that usage, while also helping prevent unauthorized or abusive use of the service.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 2, 2026Researched and drafted with AI assistanceReport an issue