Movable Type's TrackBack system allows peer-to-peer communication and conversations between blogs. Imagine that you write about a movie you just saw in an entry on your Movable Type-powered blog. Another MT blogger reads your entry, and wants to write an entry referencing your original post. He could just comment on your blog, but he'd like to keep the post in his own database and host it on his site.

Using TrackBack, the other blogger can automatically send a ping to your blog, indicating that he has written an entry referencing your original post. This accomplishes two things:

On your site, you can automatically list all sites that have referenced a particular post, allowing visitors to your site to read the response on the other user's blog.

It provides a firm, explicit link between his entry and yours, as opposed to an implicit link (like a referrer log) that depends upon outside action (someone clicking on the link to your entry).

You can use TrackBack for more than just communication on particular entries, however. You can also associate TrackBack pings URLs with categories in your blog. Whenever you post an entry to that category, the URLs you have associated will automatically be notified of your post. This allows remote sites to keep a repository of references to posts all around the web.

For example, if you run a site about Perl, you might want to provide a repository of links to Perl articles on other weblogs. Using TrackBack, you can allow other weblogs to ping a particular category in your own blog, whenever a new entry is posted that pertains to this category.