Referral sources are webpages which display backlinks for a website and are therefore a source of visit(s).
In web analytics tool interfaces, referral sources are displayed at domain and page levels.
Referral sources at domain level:
Referral sources at page level for Wikipedia:
For referral sources, retrieving referrer URLs allow to identify referrer domains and pages (backlinks) and to see on theses pages how the recommendations" and anchor texts are made. It is important for reputation analysis and for backlinks studying with a SEO perspective.
For privacy issues, some domains block or limit informations transmission related to referral sources. Thus, a Facebook referrer does not allow web analytics tools to identify page owner.
Classic SERP are referrers but are not considered as referral traffic by analytics tools. They are considered as search traffic.
Thus, referral sources include pages with backlinks, webmail pages - newsletter clicks in webmail interface ares considered as referral sources- and Google Image SERP.
An exemple of webmail referral source: