The affiliate marketing is a relationship between advertisers and users. Here it is noted that, the purpose of affiliate marketing is to drive sales. Affiliates can use a type of regular  advertising  methods for selling any products of others(Merchants). The advertising methods may include organic search engine optimization (SEO), paid search engine marketing (PPC - Pay Per Click), e-mail marketing, content marketing and in some sense display advertising. Website Owners(are referred as Affiliates) are used to make money by generating sales, leads and traffic for the Merchants business(selling products) through their websites. Now, I have specified a very familiar affiliate programs here below:

Cost Per Sale (CPA) 
A visitor is referred by the Affiliate to purchase goods(products) and services from the Merchant. This payment structure is known as Cost Per Sale or Cost Per Acquisition (CPA). 
Cost Per Lead (CPL) 
A visitor is referred by the Affiliate and filled a form on the Merchants web site. This payment structure is known as Cost Per Lead (CPL). 
Cost Per Click (CPC)
A visitor while visiting an Affiliates website may click on a Merchant's banner and visits the Merchant's website. This payment structure is known as - cost per click (CPC).

What is Ad Network?

An advertising network or ad network is a commercial and technical online advertising company that act as a bridge between Advertisers and Publishers. Here it is clear that the ad network helps the advertisers to advertise their goods (products) into a number of  web sites by its publishers. The key function of an ad network is to have an aggregation of accepting ad spaces from publishers and matching them with advertiser's demand.

There are three types of Ad Networks namely Vertical Networks,       Blind Networks,      Targeted Networks.

An advertiser can buy a specific run of network package, or a specific run of category package in the network. The advertising network serves their advertisements from ad network's central ad server, and the served advertisements have responds to a site when a page is called. That is, when a snippet of ad code is called from the ad server, that represents the advertising banner