A backlink, also called inbound link, is a link that leads from another website to your own. The number of backlinks decides the link popularity of a website.

Search engines see backlinks like a kind of reference. The more references a website has, the better it should be positioned. In the early days of search engines this caused a lot of spam from some website optimizers who only tried to to put as many links as possible on their own website. This is how link farms were created and people were selling links, trying to manipulate the results in search engines.

Since search engines improved their algorithms a lot and they can now evaluate a backlink’s reputation much better. If a search engine recognizes the use of prohibited link building methods, they will penalize the website’s ranking.

Reasonable cross references with other trustworthy domains however can improve your ranking.