In a nutshell, TrackBack was designed to provide a method of notification between websites: it is a method of person A saying to person B,
"This is something you may be interested in." To do that, person A sends a TrackBack ping to person B.
A better explanation is this:
* Person A writes something on their blog.
* Person B wants to comment on Person A's blog, but wants her own readers to see what she had to say and be able to comment on her own blog
* Person B posts on her own blog and sends a trackback to Person A's blog
* Person A's blog receives the trackback, and displays it as a comment to the original post.
This comment contains a link to Person B's post