The referer (sic) header
The referer header, misspelt for historic reasons, indicates the
link that "referred" the user to the page being requested. In some cases, the referral
may be implicit (for example, a request for an embedded image within a page will
generally send the URL of the parent page in many browsers).
The referrer string is most often used to gather
statistics about traffic sources to help with site marketing
or identifying new markets.
Remember that the referrer string contains the whole URL, so if
you want to collect statistics per site, then you will need to perform
the appropriate filtering of the URL.
Determining search terms
When the referrer is a search engine results page, many search engines also
send the search terms entered by the user. For example:
A simple regular expression will
suffice to pull out the raw search term. This could then be used to present the
user with a more appropriate use of the page, for example.
In interesting use of the referer header is as an approximate
form of geolocation: that is, guessing the country of a given client.
See the separate page on IP location
from the referrer string.