parasites and PBNs are 2 different things and I don't think there's any connection between them (at least I'm not seeing it...)
Parasites are sites - usually web 2.0s, which means sites that allow you to create a free account on them so you can publish content on them for free - that have high authority and / or rank very well in google because google likes them.
And since they rank very well you can take advantage of their high authority and love with google to publish content and insert your links inside of that content and when your content (on these parasites) ranks and gets traffic you get clicks on the links you've inserted into this content and those links will take people to whichever site you want (your money site, your affiliate offers, your YT video, your twitter account, etc...)
PBNs are a totally different thing, but I'm not in mood of explaining at the moment, maybe someone else can finish my explanation...