Even though in practice they are getting closer all the time, in the original design a Trackback ping said “I wrote something about your post that I think your readers would like to see, here’s an excerpt from it to tease them in”, while a Pingback ping said “I linked to you.”
If all blogware writers were reasonable and supported both, your linkdump posts that are just two dozen links without commentary would Pingback, and your thoughtful explanation of why someone’s crazy as a loon would Trackback (and would send a hand-written excerpt, not a cut off in midsentence excerpt of a few words of throat-clearing). But since b2’s children and a few individuals support both, a few individuals support only Pingback, and MT supports only Trackback, you end up needing both to have a chance of getting something through.
