The five parameters
The core tags are utm_source (where), utm_medium (the channel type), and utm_campaign (which campaign), plus optional utm_term and utm_content. Consistent naming is everything: sloppy or ad-hoc UTMs turn your analytics into noise that no report can fix later.
UTMs and audience building
Because a UTM lives in the landing URL, a website custom audience rule can match on it (URL contains utm_campaign=x), which lets you split one page into separate retargeting pools by campaign. That is why UTM discipline is not just a reporting nicety, it is what makes intent-level retargeting possible.
Common questions
What are the main UTM parameters?
utm_source, utm_medium, and utm_campaign are the essentials, with utm_term and utm_content available for finer detail. Consistency in how you fill them is what makes them useful.
Can UTMs be used for retargeting?
Yes. Because UTMs sit in the URL, an audience rule can match them, letting you build separate retargeting audiences per campaign even when they share a landing page.