utm_source — Traffic source (Where). e.g., google, facebook, newsletterutm_medium — Marketing medium (How). e.g., cpc, email, social, referralutm_campaign — Campaign name (Why). e.g., spring_sale, new_product_launchutm_term — Search keywords (used mostly in paid search advertising)utm_content — Content differentiation (useful for A/B testing variations like header_link vs footer_link)_ (underscore) or - (hyphen) instead of spaces.