Direct Answer
utm_source is the platform sending traffic (google, facebook, newsletter). utm_medium is the channel type (cpc, organic, email, social). utm_campaign is the specific campaign or initiative (spring-sale-2026). All three together let GA4 segment performance precisely. Add utm_content for ad/link variant and utm_term for keyword.
Voice answer (≤30 words)
“UTM source is the platform like google or facebook. Medium is the channel type like cpc or email. Campaign is the specific initiative.”
Use lowercase, hyphen-separated values. Mix-cased and inconsistent values break segmentation in GA4.
Don't tag organic Google or direct traffic with UTMs — Google's auto-tagging handles those.