A backlink is, seen from your own site, an incoming link from another website. Google writes in its explanation of ranking that, when assessing quality, it looks among other things at whether other prominent websites link to or refer to the content, and calls that generally a good sign that the information is trustworthy. A backlink is a signal; it does not guarantee a higher position.
Example
A link from a trade association’s website to your business page is an ordinary backlink. If you paid for a link, for example as part of a sponsorship, mark it with sponsored or nofollow, as Google’s link guidelines ask:
<a href="https://www.example.com" rel="sponsored">De Spaak Bike Shop</a>Links that visitors post themselves, such as on a forum, get ugc or nofollow.
Common mistake
Buying backlinks or swapping them in bulk to rank higher. In its spam policies, Google calls creating links mainly to manipulate rankings link spam. Examples Google gives: paying money for links, excessive link exchanges and links created by automated programs.
Where does a good backlink come from?
From a site that genuinely has something to do with your business: a supplier, a partner, a local club or an article that mentions your work. How to work on that is explained under link building.
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