Events in GA4 are actions users take on your site or app that you want to measure. Every interaction is an event: a page view is an event, a button click is an event, a form submission is an event, and a purchase is an event. GA4 abandoned the old hit-type structure (pageviews, events, transactions) for a single event model where everything flows through the same pipeline. GA4 automatically collects certain events without any setup: page_view, scroll (when someone scrolls 90% down), click (outbound links), video_start, video_complete, file_download, and session_start. These are called automatically collected events. You also get enhanced measurement events if you toggle that setting on, which adds form interactions, site search, and video engagement without touching code. Recommended events are Google's pre-built templates for common actions like add_to_cart, purchase, sign_up, or generate_lead. Use these exact naming conventions because they unlock specific reports and integrations in GA4. Custom events are anything else you define: tracking a specific CTA click, a chat widget open, or a calculator interaction. Each event can carry up to 25 parameters (bits of extra data). Parameters let you pass product IDs, transaction values, content categories, or any dimension that adds context. The event name tells you what happened; the parameters tell you the details. At Ottawa SEO, we set up custom events for high-intent actions that standard tracking misses: PDF downloads of service guides, clicks on phone numbers in the header, or interactions with cost calculators. We avoid over-eventing because GA4 has a 500-event-name limit per property and processing limits on custom dimensions. Focus on events that tie to conversion paths or help you understand user intent, not every possible micro-interaction. If you can't imagine using the data in a report or audience segment within 90 days, skip the event.