What is (Private blog networks) PBNs?

PBN is the best way to increase your site traffic.

A private blog network (PBN ) is a set of domains that you or another individual owns. It is possible to have a PBN consisting of free blogs, like wordpress.com, tumbr.com, or livejournal.com. Typically, the free blog domains do not have as much power as self hosted blogs.

A Private Blog Network is a network of authoritative websites used to build links to your money website(s) for the purpose of ranking higher in the Google search engine.

A blog network, also known as a link farm, is a group of blogs that are owned by the same entity. A blog network can either be a group of loosely connected blogs, or a group of blogs that are owned by the same company. ... In September 2014, Google targeted private blog networks (PBNs) with manual action ranking penalties.

PBN is private blog network. Its a internet marketing strategy to boost your blog or website traffic.

PBN is the best way to increase your site traffic.

Source: lionzeal
A Private Blog Network (PBN) is a network of authoritative websites used to build links to your money website(s) for the purpose of ranking higher in the Google search engine. A money website is the website you intend on ranking i.e. the one that actually makes money. This can also refer to a clients website.

Hope this answers your question: Google Hits PBNs: Google Slapped Private Blog Networks

I have No Idea How they are harmful or hrlpful for website ranking? Please help.