Trackbacks were originally developed by SixApart, creators of the MovableType blog package. It’s a notification method between websites working as follows :

• Person A writes something on their blog.
• Person B wants to comment on Person A’s blog, but wants her own readers to see what she had to say, and be able to comment on her own blog
• Person B posts on her own blog and sends a trackback to Person A’s blog
• Person A’s blog receives the trackback, and displays it as a comment to the original post. This comment contains a link to Person B’s post

Most trackbacks send to Person A only a small portion (a teaser called an “excerpt”) of what Person B had to say. One problem is that there is no actual verification performed on the incoming trackback, and indeed they can even be faked.

Pingbacks were designed to solve some of the problems of trackbacks. The official pingback documentation is available on the website www.hixie.ch.

The best way to think about pingbacks is as remote comments:

• Person A posts something on his blog.
• Person B posts on her own blog, linking to Person A’s post. This automatically sends a pingback to Person A when both have pingback enabled blogs.
• Person A’s blog receives the pingback, then automatically goes to Person B’s post to confirm that the pingback did, in fact, originate there.

There are two significant differences between pingbacks and trackbacks : pingbacks and trackbacks use drastically different communication technologies (XML-RPC and HTTP POST, respectively) and pingbacks do not send any content.

A useful guide “Introduction to Blogging” with more details about trackbacks and pingbacks is published by WordPress.