This was brought to my attention by Scott Leslie (thanks, Scott!).
Ryan Eby has an interesting post about the differences between Trackback and Pingback, and how they might be used in the context of learning object repositories. Ryan is commenting on a post by Phil Ringnalda.
From Ryan's post:
For example, if I find a learning object such as MLX #467 on Atomic Structure and decide I want to write a review of it or something similar, then I would want to send it a trackback so that others can find my post on it, if they are browsing the objects. However, if I actually use the flash animation in some sort of online setting (as part of a lesson, etc.) then it would probably be wiser for me to send a pingback as just a way of showing I'm using it. Pingback is structured so you can ping any file and so it probably a more sensible thing for when I'm just using an image or something for a site I made.
I had never heard of pingback before, but it sounds very interesting. Where Trackback is "Here's what I think of that...", Pingback is more simply "I'm using that..." - a useful way to track activity, similar to what is done internally in CAREO, but in a distributed manner.