event

GA4 runs on an event-based data model. Universal Analytics tracked pageviews and sessions as separate hit types; GA4 collapses everything into one shape where each interaction is an event with a name plus optional parameters that carry detail (page_location, value, currency, and so on).

The four event types

• Automatically collected fire by default, like first_visit and session_start.
• Enhanced measurement turn on at the data stream, like scroll, page_view, and file_download.
• Recommended are predefined names you implement yourself (purchase, generate_lead) that unlock extra reports.
• Custom are events you name when nothing else fits.

You send an event with the Google tag:

gtag('event', 'generate_lead', { value: 100, currency: 'USD' });

An event you flag as business-critical becomes a key event. Google renamed the old “conversions” to “key events” across the GA4 interface in 2024, and reserved the word “conversion” for the action passed to Google Ads. So the flow is: an interaction fires an event, an important event gets marked as a key event, and that key event can feed Ads as a conversion. Getting the event layer right is what makes every downstream report and attribution model trustworthy.