When visiting a webpage, the referrer or referring page is the URL of the previous webpage from which a link was followed.
More generally, a referrer is the URL of a previous item which led to this request. The referrer for an image, for example, is generally the HTML page on which it is to be displayed. The referrer field is an optional part of the HTTP request sent by the browser program to the web server.
Many web sites log referrers as part of their attempt to track their users. Most web log analysis software can process this information. As referrer information can violate privacy, some browsers allow the user to disable the sending of referrer information. Some proxy and firewall software will also filter out referrer information, to avoid leaking the location of non-public websites. This can in turn cause problems: some servers block parts of their site to browsers that don't send the right referrer information, in an attempt to prevent deep linking or unauthorized use of images (bandwidth theft).