"LinkBacks" is the generalized term we use to reference three methods of communication between Websites:
LinkBacks (Trackbacks, Pingbacks and Refbacks) allow you to notify another site that you wrote something related to what is written on a specific page. This improves the chances of contributors to this page noticing that you gave them credit for something, or that you improved upon something they wrote. With LinkBacks, websites are interconnected. Think of them as the equivalents of acknowledgments and references at the end of an academic paper, or a chapter in a textbook.
Linkbacks have long been a major force in the development of the blogging network, by creating an interconnected series of blogs and posts acknowledging one another. Not only does this improve the general community ethos throughout the "blog-o-sphere", but it also helps to make blogs into more powerful link-building tools.
Note: Links built via this method are highly relevant and do not carry the disadvantages typically associated with "link farms" or "link exchanges".
The LinkBack principle has migrated from blogs to other publishing platforms such as news content management systems and forums.
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 phisically link to Site B.
A Pingback is also a signal (ping) sent from Site A to Site B. However, it's also a link. When Site B receives the notification signal, it automatically goes back to Site A checking for the existance of a live incoming link, if it exists, the Pingback is recorded successfully. This makes Pingbacks less prone to SPAM than Trackbacks.
Both sites must be Pingback enabled in order to establish this communication. If a site is Pingback enabled, each time you link-out you will be "pinging" external sites. Pingback requires for Site A to phisically link to Site B.
A Refback is also a link. However in this case, Site A (link originator) does not need to "tell" anything to Site B (receptor). Instead, the receptor Site "discovers" this link immediately after the first web visitor gets to the site by clicking on the link. This is done by analyzing information carried by this web visitor's browser referer header.
This is an easier method than Pingbacks since the Site originating the link doesn't have to be Pingback enabled (Posting a link back within any webpage is good enough).