An authority site is determinded the Number and quality of links that site has and the quality of content that site has,

If the site has lots of quality contents it attracts lot of quality inbound links and this will make that site an authority site,

For example microsoft is an huge authority site for computer related questions and windows related doubts, they get backlinks for most of their inner pages from various sources which makes them an authority,

They just dont get links to their homepage but to their 100s of 1000s of inner pages too, Simply saying an authority site is an great source of information which attracts all sort of crowd,

SEO Blog Team,

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