Webhook
A webhook is an automated message one system sends to another the instant an event happens, an HTTP call triggered by the event itself, rather than a schedule someone has to remember to run or a person manually exporting data.
Most system-to-system connections in a GTM stack work one of two ways. One system can poll another on a schedule, checking every few minutes whether anything changed, which wastes calls and still leaves a delay between the event and the check. Or the system where the event happens can push a message the instant it occurs: a reply lands, a form gets filled, a record updates. That push is a webhook. It's an HTTP request the sending system fires automatically, carrying just enough data about what happened for the receiving system to act on it right away.
Why it matters more than it sounds
A webhook is what lets an enrichment table update the moment a workflow tool finds a new field, or a CRM record update the moment a reply comes in through a sequencer, without a person exporting a spreadsheet or clicking a sync button. Custom, webhook-driven signals are also how teams build first-party intent triggers themselves instead of buying a packaged data product: a website event, a product-usage spike, a support-ticket pattern, wired to fire into whatever workflow tool is running enrichment and outreach. The tradeoff is that someone has to build and maintain that wiring, watch it for drift, and fix it when a source changes its own API, work a packaged vendor product would otherwise absorb.
In practice
The practical test for whether something needs a webhook: does a person currently notice an event happened and then go do something about it by hand? If yes, that manual noticing step is exactly what a webhook replaces, wiring the event straight into whatever should happen next.
What people get wrong
People assume a webhook and an API call are the same mechanism running in opposite directions, and mostly that's right, but they get the trigger backwards. An API call is something a system requests when it wants data. A webhook is something the other system sends without being asked, the moment its own event fires. Building a custom webhook layer also isn't free just because there's no subscription attached to it. The engineering and maintenance burden lands on whoever built it, not on a vendor.
