PBN tends to be a small group of websites grouped to boost the rank of a single website by linking each other; it can be thought of as a pyramid scheme due to how smaller websites are used to boost the ranking in a search engine for a bigger website.
Private Blog Networks are a forbidden practice because they can easily hurt a website’s rank in a search engine if search engines catch the links produced by the PBNs.
It is a great way to get a website penalized thanks to Google and other search engines frowning upon this practice; it was a controversial topic way back when it was more popular. It is a list of sites without related content linking to one primary domain to promote its link authority.
The common modus operandi of Private Blog Networks is buying old domains with a lot of authority in the search engine and producing new content with them while linking the primary website in their content, therefore boosting its link authority.
That way, the primary site will get a good amount of quality backlinks without too much effort. Their standing in the search engine ranking will be boosted.
Private Blog Networks are not recommended and are Illegal due to Google’s Guidelines. Websites discovered involved in Private Blog Networks will be penalized and have all of their rankings in the search engine removed; it is a method that had fallen from grace when it was hailed as something to be proud of, now it is regarded as a black hat method that should be avoided at all cost, lest the consequences fall upon them.
Even so, Private Blog Networks still are being practiced in this day for the simple reason that they work and produce results, thanks to Backlinks being one of the primary factors that Google uses to rank web sites and thanks to the quality backlinks gained with the constant linking from Private Blog Networks, websites are easily boosted due to the way backlinks tell Google that a website has quality content and that it has value, that way maintaining and boosting rank can be an easy thing to do with backlinks.
Something to always have in mind is that Google is always looking for Private Blog Networks. The risk of being caught is not worth the effort; losing all the website ranking can be a tough hit for a webmaster.
The way a private blog network looks tends to be simple and straight forward; it is a small amount of old or new websites with high popularity producing new content constantly and linking a central domain to make it more popular, to boost its ranks on the search engines and make it more visible.
It is similar to pyramidal schemes. Small websites build the central domain pillars, making them gain quality backlinks and traffic that it wouldn’t have gotten any other way.