Trackbacks seem to be unnecessarily confusing. A trackback is a reference on one website (usually a blog) to another website (ditto). MovableType supports trackbacks fairly intuitively, but a lot of blogging tools don’t.
Simpletracks allows either party to create a manual trackback. To do this, you need to know two things: the permanent URI of the entry with the reference, and the trackback URI of the entry being referred to.