Some people still think a link is a link, is a link and that any backlink pointing to your website is of the same value.  Nothing could be further from the truth.  Google has complied extensive data about all websites on the web.  This data creates a link profile and based on that, weighs a certain amount of power on the link itself.  So what does that mean?

Understanding Linking Relationships

In the early days of Google when they still had Pagerank links were more universal.  If a site with a Pagerank of 6 linked to you, it had the same power for your site as it would with any other regardless of topic or niche.

Now with new algorithms and a better understanding of the web, Google has weighted these links with new metrics.  Although all Google algorithms are highly secretive, other SEO companies have tried to replicate them using metrics that they call Page Authority or Trust Authority, etc.