TrackBack is a relatively new feature in the weblogging world, but it is a natural extension of the medium when you think about it.
Though most weblog tools have a “comment on this item” feature, the response to an item is more likely to appear in the responder’s weblog. Before TrackBack, such discussion would remain amorphous and distributed until a robot program came along and assembled the pieces from across the Internet. TrackBack allows me to cross-post my comment to both of our servers, eliminating the middle man (though in the weblogging world, middle men are often a good thing).
At first this scheme might seem kludgish, but it has some fairly interesting characteristics. For example, my weblog URL now serves as a kind of digital identity. Because I am authenticated by my URL (thank you DNS), I can now post messages to all kinds of places without having to sign in at each one. Nor do I have to learn a new set of user interfaces for each one. I simply post to my own blog and ping the appropriate TrackBack URL.