The TrackBack specification is due to Six Apart who first implemented it in its Movable Type blogging software in 2002. It has since been implemented in most other blogging tools. Six Apart started a working group in February 2006 to improve the Trackback protocol and have it potentially approved as an internet standard by the IETF. One notable blogging tool that does not support trackback yet is Blogger. (Blogger now has backlinks - very similar to the trackback feature in Movable Type.)
A Trackback is simply an acknowledgment. This acknowledgment is sent via a network signal (ping) from Site A (originator) to Site B (receptor). The receptor often publishes a link back to the originator indicating its worthiness. Trackback requires both Site A and Site B to be Trackback enabled in order to establish this communication. Trackback does not require for Site A to physically link to Site B.
Trackbacks are used primarily to facilitate communication between blogs: if a blogger writes a new entry commenting on, or referring to, an entry found at another blog, and both blogging tools support the TrackBack protocol, then the commenting blogger can notify the other blog with a "TrackBack ping"; the receiving blog will typically display summaries of, and links to, all the commenting entries below the original entry. This allows for conversations spanning several blogs that readers can easily follow.
See also: referrer
Blogging software that supports the TrackBack protocol displays a "TrackBack URL" along with every entry. This URL is used by the commenting blogger, whose software will send XML-formatted information about the new entry to this URL. Some blogging tools are able to discover these TrackBack URLs automatically, others require the commenting blogger to enter them manually.
Some individuals or companies have abused the TrackBack feature to insert spam links on some blogs (see sping). This is similar to comment spam but avoids some of the safeguards designed to stop the latter practice. As a result, TrackBack spam filters similar to those implemented against comment spam now exist in many weblog publishing systems. Many blogs have stopped using trackbacks because dealing with spam became too burdensome.