A Private Blog Network is a network of websites, typically created or controlled by the same entity, for the sole purpose of linking out to a “money site” (the site you want to rank). PBN sites are usually made to look like independent blogs or sites, but they exist only to host content with links to the target site. PBN builders often use expired domains (with existing backlink profiles) to set up these blogs so they have some authority from the get-go. They then publish articles (often low-quality or spun content) on these blogs containing links to their main site (and sometimes to each other to appear legit).