Try this. You'll find more info than you probably want/need.

It's a network of privately owned websites, built for the purpose of building backlinks to one or multiple money sites.
You would make one by buying up a bunch of domains (preferably aged with good authority) and building them up as unique websites themselves. Optimally each network site would need to be hosted on seperate IPs to each other. This would just be a basic idea. There are other factors that We won't go into. As the above poster mentioned. Use the search function. There are hundreds of threads that speak about it.

Be careful of blog networks...