Trackbacks: A system by which blogs may post messages to each other about citations and related resources. Not all blog software supports Trackbacks, though it may have a system that operates in a similar fashion.
Example: Blog A writes an article. Blog B likes the article and mentions it in their most recent post. Blog B uses a trackback link when referencing the article. The trackback link pings Blog A to let it know that it has been referenced. Now Blog A will show a link to Blog B that can be indexed by search engines, along with an excerpt of your post.