User Property

A user property is an attribute that describes a user rather than a single action.

Where an event describes a moment, a user property describes the person across their whole relationship with your site, like their membership tier or preferred language. You set it once and GA4 applies it to that user's data going forward.

You set a user property called membership_level to "premium." From then on you can compare how premium users behave against everyone else, no matter which events they fire.

User properties are not retroactive and carry privacy weight. They apply from the moment you set them, never to past data, and you must not store personally identifiable information like names or emails in them. That breaks Google's terms.

Events describe the click. User properties describe the person.