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What is an API?
An API is a set of rules that lets one piece of software ask another piece of software to do something.
- Category Building with AI
- Also called APIs, application programming interface
- Updated 20 Oct 2026
API, explained simply
API stands for application programming interface. It is how apps talk to each other.
It matters because most AI tools use one. When an app calls an AI model, it sends your request through an API and gets an answer back.
Think of a waiter in a restaurant. You tell the waiter your order, and the waiter takes it to the kitchen. The API is the waiter.
Developers use APIs to add features without building everything themselves.
On ITG Now, our free tool guides say when a tool needs an API and how to connect it, step by step.
Related terms
API keySDKRate limitJSONWebhook
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Questions people ask about API
Is an API free?
Some are free and some charge. AI model APIs usually charge by how much you use.
API vs SDK: what is the difference?
An API is the set of rules for talking to a service. An SDK is a toolkit that makes using those rules easier.
Do I need to know about APIs to use AI tools?
Not for most apps. You only need to know about them when a tool asks for an API key.