An affiliate network acts as a go-between between you (the publisher / affiliate marketer on your website) publishers and the merchants who create or manufacture those products and services offered through their affiliate programs.

For you the affiliate marketers, an affiliate network provides a “listings” of companies offering affiliate marketing programs.  They are typically organized by category and provide you with a simple way to join and set up those programs. 

While you, the affiliate marketer will be able to join these affiliate networks for free, the merchants usually have to pay a fee to participate in the network.