How come on affiliate networks, we are "publishers", but on traffic sources like popads, we are "advertisers"?
because in aff. network you sell traffic, but in traffic source resources you buy traffic - it's easy
Mind blown! Thank you!
Yup, it's pretty easy but many people get confused by this when first entering the AM biz 
Publisher is the one who has traffic. Advertiser is the one who wants traffic.
So on a traffic network, publishers are the owners of sites and advertisers are those who are buying traffic from them.
In the case of an affiliate network, the principle is the same, advertisers are those who want the traffic and publishers bring it. But since you already bought the traffic in a traffic network, you are now becoming the publisher 