What it means
An API (application programming interface) is the public-facing door of a software system. Your AI agent says 'give me this customer's order history' and the API responds. APIs are pull-based: the agent decides when to ask.
A webhook is the inverse. The other system fires an HTTP request at your agent when something interesting happens: a payment lands, a form is submitted, an appointment is cancelled. Webhooks are push-based and event-driven. The agent does not poll; it just listens.
Most production AI workflows use both. The agent reacts to webhooks for real-time triggers and calls APIs to enrich the response.
Why it matters
Without APIs, AI agents have to read screens or scrape pages, which is brittle. Without webhooks, they have to poll for changes, which is slow and expensive. Together they are how a modern AI deployment becomes both responsive (under a second to react) and accurate (always reading current data).
It is also worth saying out loud: any system worth integrating with publishes APIs and webhooks. Anything that does not is a flag for technical debt downstream.
Example
A clinic books an AI appointment-confirmation agent. A webhook from the booking system fires the moment an appointment is created. The agent calls the CRM API to fetch the patient's preferred language and channel. Then it sends the right confirmation in WhatsApp or SMS, all under 800 milliseconds end to end.