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).

Thanks sumicubi,actually this topic is new for me and i never heard about it so thanks for the wonderful information.