I've just learned by myself how to use these "trackbacks", which literally are links to other weblogs which helps to find comments to a bog entry made in an other weblog. So e.g. Michael Marleau writes something about an old steamboat from 1880 and I'd like to add my five-cents to that. I can do this either by posting a comment on Michael's blog directly, or I post something in my own blog and put a trackback link in the appropriate box at the bottom of the "new entry" form. As soon as i do this, everyone reading Michael's posting will see that there is a trackback and can click on the link to see my posting related to Michael's blog.
So, where to get this "trackback link"? When you read the original posting, you'll find a link called "Trackbacks" below th posting. Click on this link and you'll find a link called "Trackback specific URI for this entry". Right-click on it and choose "Copy Shortcut". Now you can paste this to the "Additional Links to Trackback" box on your "new entry" form - done.
I'm doing this for demonstration in this posting, too, using the trackback link to Michael's posting about the quarantine in St. Louis.