An API (Application Programming Interface) is a set of defined rules and protocols that allows one piece of software to interact with another. It specifies what requests can be made, how to make them, and what format the response will come in.
When a weather app on your phone shows the current temperature, it’s calling a weather service’s API to request that data. Your phone doesn’t generate the forecast — it asks for it through the API, gets a structured response, and displays it.
You’ll hear this when…
APIs are fundamental to modern software. If someone mentions “calling an API,” “API integration,” or “the API is down,” they’re talking about software systems exchanging data. Nearly every web and mobile application relies on APIs to function.
“REST API” is the most common type — it uses standard HTTP methods (GET, POST, PUT, DELETE) and returns data in JSON format. “GraphQL” is an alternative that lets the client specify exactly which data it wants.
In business conversations, “do they have an API?” is a question about whether a service can be programmatically connected to other tools — a key factor in software purchasing decisions.
Public vs. private APIs
Public APIs are available to external developers (Stripe’s payment API, Twilio’s messaging API). Private APIs are used internally within a company to connect its own systems. Partner APIs sit in between — available to authorised third parties under specific agreements.
One practical concept newcomers encounter quickly: rate limiting. Most APIs restrict how many requests you can make in a given time period (e.g., 100 requests per minute). Exceed the limit and the API returns an error instead of data. Rate limits protect the service from being overwhelmed, but they can surprise developers who aren’t expecting them.
Source: MDN Web Docs — introduction to web APIs