A Private Blog Network is a group of sites under your full control that host niche-relevant articles and link back to your money sites. PBNs require a massive amount of work and money to set up correctly. Then there are recurring maintenance costs to keep them hidden and up to speed. The goal is not to associate the blogs with others in the group in any detectable way. When done right, PBNs can offer huge gains; done wrong and will get de-indexed (money site receives a penalty too) or hacked.