What is trackback?

A friend of mine just asked me this question. So as I've spent 10 writing a quick description in an email, I might as well post it here.

The original idea of trackback is to help find out who links to you.

Your blog publishes a URL which I can "ping", ie. which alerts your blog to the fact that I have a post on mine that links to it.

Trackback is a kind of automation of the practice of posting a comment on someone's blog saying "You're wrong. And I responded over at .... "

Normally, the blog which receives the trackback shows a short snippet of the responding post.

I think it's still heavily used, but might be falling out fashion due
to the blog search engines. If I respond to you on my blog I still
need to find your trackback URL and ping it, often manually. OTOH, as you're already probably using Technorati etc. to find people who link to you anyway, maybe I don't need to bother.

I could be wrong about this. And trackback is possibly still playing an important (and increasingly popular) part in the ecosystem. I have no statistics to back me up. It's just that people aren't "talking" about trackback much ... OTOH maybe it just works :-)