Ok, I found a resource that answers "my what is a trackback" question:
A Beginner's Guide to TrackBack
"This document is an introduction to TrackBack from a non-technical perspective. The goal is to illustrate how the system can be used to enhance cross-site conversations and build community."
Basically, it's a way to automatically notify another site that you commented on that site. Or, it can be used to notify a central site (such as a directory or search engine) that you've updated your site.