A referrer is the URL of the webpage a visitor was on immediately before navigating to the current site, typically passed by the browser via the HTTP Referer header.
Referrer data is useful but incomplete: browsers omit or truncate the Referer header in many situations for privacy reasons, direct traffic (typed URLs, bookmarks) carries no referrer at all, and most mobile apps strip it entirely when opening links — which is why a large share of traffic in most reports ends up bucketed as 'direct' even though some of it genuinely came from somewhere. Analytics tools group referrer data into broader traffic-source categories, like search, social, or other websites, to make it usable.
A visitor clicks a link to a blog post from a Twitter timeline; the analytics platform records twitter.com as the referrer and typically classifies the visit under 'social' traffic.
Direct traffic includes genuinely typed URLs and bookmarks, but it also absorbs traffic where the referrer was stripped — by HTTPS-to-HTTP transitions, some mobile apps, or privacy-focused browsers — so 'direct' is often a mix of true direct visits and lost referrer data.
Not on its own — referrer only shows the last page a visitor came from, not which specific campaign or ad drove the click, which is why marketers add UTM parameters for more precise tracking.
No — referrer policies vary by browser and site configuration, and some browsers or privacy extensions block or limit the Referer header, especially for cross-site navigation.