The Referer header from the request. (It is misspelled in the HTTP spec, and thus the actual request and cgi specs too, but I spelled the word correctly here because that's sane. The spec's misspelling is an implementation detail.) It contains the site url that referred the user to your program; the site that linked to you, or if you're serving images, the site that has you as an image. Also, if you're in an iframe, the referrer is the site that is framing you.
Important note: if the user copy/pastes your url, this is blank, and, just like with all other user data, their browsers can also lie to you. Don't rely on it for real security.