A pingback is one of four types of linkback methods for Web authors to request notification when somebody links to one of their documents. This enables authors to keep track of who is linking to, or referring to their articles.
Open graph description entity tag
A scheme.org entity that represents an organization such as a school, NGO, corporation, club, etc.
A scheme.org entity that represents a person (alive, dead, undead, or fictional).
Sites who has this tag are verfied by google to use WebMaster tools
A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical", or "preferred".