"LinkBacks" is the generalized term we use to reference three methods of communication between Websites:
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.
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.
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.