A link network is a service allowing webmasters to exchange or buy backlinks to improve SERP rankings.
More visible and exposed link networks are less and less effective and more and more dangerous since Google is fighting back this type of backlink offers.
Google can de-index or penalize the link network website and also penalize websites selling or buying links when Google’s guidelines violation is blatant.
Private and more discreet niche link networks are more difficult to detect and unmask and can likely still have some results on SERPs.
An exemple of link network offer.