Referrer field

The referrer header field (mistakenly spelled “referer” in the HTTP standard) is a unit of information that contains the URL of the site you are currently in. The referrer header field is sent automatically to any site you are about to visit when clicking a link. Referrer headers allow reading patterns to be studied and reverse links drawn. The address of the page might contain privacy information (such as your name or email address), or might reveal personal interests that you would rather keep private.