API stands for Application Programming Interface. It's a set of conventions by which software programs request services from one another and exchange data.
If you see a piece of software as a company, an API is like its service window for the outside world. External systems don't need to know how the company operates internally—they just submit requests according to the window's rules and get the corresponding results.
Grab It in One Sentence First
An API is an interface through which software programs, following agreed-upon conventions, call each other's functions, read data, or submit operations.
There are many everyday examples. When you place an order in a food-delivery app, you don't need to know how the restaurant kitchen cooks; the app just sends the order out through an interface and receives the status back. When you use third-party login, maps, payments, and SMS notifications on a website, none of it can do without APIs.
The Problem APIs Solve
Modern software is rarely an island. A website may need to call a payment system, a map service, an SMS service, a login service, an inventory system, and an AI model service. Without APIs, every system would have to understand the others' internal structure directly, making maintenance extremely difficult.
flowchart LR
App["Application A"] --> Request["API request"]
Request --> Service["Application B / service"]
Service --> Response["API response"]
Response --> AppAn API wraps a complex system into a callable entry point. The caller sends a request, and the service returns a response. The request contains parameters, such as a user ID, page number, search term, or order information; the response contains results, status, or an error description. To prevent unrestricted access, many APIs also require a key, login credentials, or another authentication method.
Common Forms of APIs
The most common is the Web API—an interface accessed over the network. REST API is a very common style of it, usually using a URL to represent a resource and an HTTP method to represent reading, creating, updating, or deleting. GraphQL, on the other hand, lets the caller declare exactly which data it needs, avoiding getting too much or too little at once.
SDKs are also closely related to APIs. An SDK usually wraps API calls into a toolkit in a certain programming language, so developers don't have to hand-write the low-level requests. To the user, an SDK is more like a "handy tool," while the API is more like the service entry point behind it.
Its Relationship to AI
AI applications often need to call APIs. A chat assistant checking the weather, creating a calendar event, querying an order, reading a CRM, sending a message, generating an image, or generating vectors may all be done through an API. For an Agent, an API is one of the key channels for going from "being able to talk" to "being able to get things done."
OpenAI, maps, payments, SMS, databases, and enterprise systems usually all provide capabilities through APIs. Behind protocols like MCP, existing APIs are often wrapped into a form more suitable for AI applications to call.
Where It's Easy to Misunderstand
An API isn't a web interface. A web interface is mainly for people to click buttons, while an API is mainly for software to call. Having an API doesn't mean it can be accessed freely either; most APIs have permissions, quotas, and billing rules.
Another misconception is treating an API as the database itself. An API is an entry point for accessing functions or data, and behind it may be a database, a business system, a third-party service, or a model service. A successful call doesn't mean the business succeeded either—you also have to look at the returned content, the status code, and error handling.
How to Decide Whether to Use It
When using an API, pay attention to permissions, rate limits, error handling, data formats, and security. Keys can't be leaked, sensitive data must be protected, and failed requests must be retryable or give a clear prompt.
If an API will perform actions like writing, deleting, paying, or sending messages, it especially needs a confirmation mechanism and logging.