Damien’s got a good definition of a trackback. I think it’s also interesting that the trackback is on the blog/CMS of the comment author because that does two important things: first, it advertises the other blog and second it creates a different dynamic for censorship - as only the trackback link can be deleted, not the comment.
update
With trackbacks and comments making debate very lively, it is interesting news that the NY Times Op-Ed page will not be available for free any more (with many saying they will not link to it anymore). Looking at the discussion of the Balgaric affair and Hugh Brown’s comments I link to in this entry I would have to say that any media would be foolish to try and make gated enclaves. But perhaps the NYT will become a more common example of the sucessful gated community model as seen in The WELL.
I think I have to reconsider my copious linking to The Age also, as it requires registration. I don’t notice these days because I have the bugmenot firefox extension. Well worth checking out.