What is an API? (Application Programming Interface)

Every time you check the weather on your phone, pay online, or sign in to an app with your Google account, an API is quietly doing the work in the background. APIs are how different pieces of software talk to each other, and they are what make today's connected apps possible.

This guide explains what an API is in plain language, how APIs work, the main types, everyday examples, how apps use AI APIs from models like GPT and Claude, basic API security, and how to start using one yourself.

An API, short for application programming interface, is a set of rules that lets one software application request data or services from another. It defines what you can ask for, how to ask, and what format the answer will come back in, so developers can use another system's features without knowing how it works inside.

The classic analogy is a restaurant waiter. You, the customer, do not walk into the kitchen; you give your order to the waiter, who passes it to the kitchen and brings back your meal. An API plays the waiter's role: your app sends a request, the API delivers it to the system that has the data, and it returns the response in a predictable format.

Most APIs follow a simple request-and-response pattern. A client, such as your phone's weather app, sends a request over the internet to a server. The server checks that the request is allowed, does the work, and sends back a response, usually in a lightweight data format called JSON. A typical request is made up of a few parts:

Because the rules are fixed and documented, any developer can build on an API, and the provider can change what happens behind the scenes without breaking the apps that use it.

Public APIs, also called open APIs, are available to outside developers, sometimes free and sometimes for a fee. Private APIs are used only inside a company to connect its own systems, while partner APIs are shared with specific business partners under an agreement. Many apps also use composite APIs, which bundle several calls into a single request to save time.

APIs also differ in how they are designed and how they exchange data.

REST is by far the most common today, which is why "API" in everyday conversation usually means a REST API.

You use dozens of APIs a day without noticing. A weather app pulls forecasts from a weather service's API. "Sign in with Google" and "Sign in with Apple" buttons rely on those companies' login APIs. Online stores process payments through APIs from providers like Stripe or PayPal, ride-hailing apps embed maps through a mapping API, and travel sites compare flights by querying airline and booking APIs. Even many AI chatbots are built by connecting to AI models through their APIs.

Some of the most important APIs today give developers access to large language models. Instead of training their own AI, companies send text or images to an AI provider's API and receive a generated answer. OpenAI, Anthropic, and Google all offer APIs for the models behind ChatGPT, Claude, and Gemini, and they usually charge based on how much text is processed, measured in tokens.

AI APIs are also the foundation of AI agents, which call many different APIs to search the web, send emails, or update records on a user's behalf. Standards such as the Model Context Protocol build on top of APIs to make those connections easier for AI applications to discover and use.

Chat Smith is a good example of an app built on top of AI APIs. Rather than offering an API of its own, this free AI chatbot for Android, iOS, and the web connects to models from several providers, including GPT-6 Sol, Claude Sonnet 5, and Gemini 3.5 Flash, so everyday users can work with all of them in one place without signing up for developer accounts or managing API keys. Free users get limited use, and Chat Smith Pro raises those limits for $9.99 per month or $39.99 per year.

Because APIs open doors into systems, security matters. Well-built APIs require authentication, using API keys, OAuth tokens, or similar methods, so only authorized apps can connect. They encrypt traffic with HTTPS, validate every input to block injection attacks, and apply rate limits to prevent abuse. If you are using an API, treat your API key like a password: never share it publicly or paste it into code that others can see.

Getting started is easier than it sounds. Pick an API with good documentation, sign up for a key, and read how its endpoints and parameters work. Tools like Postman or a simple command-line request let you test calls and see the responses before writing any code. From there, you can call the API from a programming language such as Python or JavaScript.

If you get stuck, dedicated coding assistants can explain documentation, draft a first request, or help debug an error message, a practical form of natural language programming. Our guide to the best AI chatbots for engineering compares the strongest options for technical work.

APIs let software systems share data and features without needing to know how the other side works, which is why almost every app you use is built on them. From payments and maps to logins and AI models, they turn complex services into building blocks that any developer can plug in. Understanding how APIs work makes it much easier to make sense of, and build, the connected apps that shape everyday life.