Syllabus Magazine does a great review on Trackbacks for the uninitiated, oddly though they have no trackback facility.
TrackBack: Where Blogs Learn Their Places | Syllabus Magazine
“Someone, let’s say Tom, has written a post on his own Weblog that comments on a post in another person’s Weblog, say Paula’s. How does Paula know or learn about Tom’s interest and reference to her post? Tom could have gone to Paula’s Weblog and posted a note there saying he liked Paula’s comment and wrote about it on his blog at http://toms_log.myblog.com. However, this takes an extra degree of effort, and is just the sort of communication barrier that keeps like-minded people from forming communities. What is needed is a form of remote comment—rather than posting the comment directly on Paula’s Weblog, Tom posts his comment about what Paula has written on his own Weblog, then he sends a TrackBack ping to notify Paula.”