MovableType describe a TrackBack as being:

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.

So why would anyone want to do that?

Well, it could be that someone has written something about someone else’s blog or a post on their blog.

Or it could be that something has been written that a group of other people may find interesting and the author wants to attract that group’s attention to the fact. In this case, we would like to let other mountain bike sites know when we have added something new to this site.

So, if you see a link listed under the Trackback panel on this site, it simply means that another blog somewhere has linked to one of our articles. They let us know by pasting the Trackback URL into the box on their blog entry that says ‘Sites to ping with this post’.

So now you know!

As a matter of course we also let www.technorati.com, www.weblogs.com and www.blo.gs know when we update the site.

More info from Movable Type