Trackback, Referrers, etc.

A Beginner's Guide to TrackBack from Movable Type, where it was invented:

“In a nutshell, TrackBack was designed to provide a method of notification between websites: it is a method of person A saying to person B, "This is something you may be interested in." To do that, person A sends a TrackBack ping to person B.”

Daring Fireball: Take Your TrackBacks and Dangle

Textism: Refer 2.0: “Refer is a web application that tracks incoming referrers”.