Google matt cutts head of seach spam has said laterly that they Rel=author can help in search quality but still Googlw is not using it again it has confirmed by John Mueller which is Google web trends analysts.

what is meant by authorship tag and what's the use of it...? please kindly clear my doubt regarding this

Some people’s faces are showing up in search results. This is done by linking and then adding the Google Authorship markup to one of the links. This is called a “rich snippet.” The steps involved are:

 •  Verify your email address on Google+
 •Link from your Google+ profile to your content
 •  Link from your content to your Google+ profile

Now i came to understand and thanks for this nice sharing

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