Attribution
Referrer
Also called HTTP referrer.
Referrer is the address of the previous page that the browser reports when it navigates to yours. It is the main evidence of where a visit came from.
How it is measured
The browser sends it in a header on a click from one page to another. The collector reads it at the start of the session. Referrer policies, apps, and HTTPS to HTTP transitions can reduce or remove it.
To check what you receive, click through from a few known sources, such as a social app and a search result, and inspect the value your tool records.
Worked example
A regional news site finds 3,100 sessions a week with a referrer from a news-aggregator app. A look at the raw values shows the aggregator sends only its domain, not the article path.
The editor cannot see which aggregator story drove traffic, so she asks the partner to add tags to their links. In the next weeks, the sessions arrive with tags that show the article and feed position.
How it differs
A referrer is the full previous address. A referring domain is just its host name. The first shows the page that linked to you; the second only the site.
Common errors
Assuming it is always present. Ignoring referrer policy changes. Treating empty as direct only. Mixing up the referrer and the tagged source. Counting internal referrals as sources.
In practice
Test your top three inbound sources and see what arrives. Exclude your own domains and payment domains from referral lists. Add tags where the referrer is stripped.