This is one question that has been bothering me for years whenever I sign up to offer sources and traffic sources. I always struggle with the concept of what bit I sign up for. Essentially the choices generally are Advertiser, Affiliate or publisher. This always seems to get my head in a spin, maybe its just me. Traffic sources are easier to understand as I am the advertiser and that is what I sign up for and I get it that I'm the middle man here but can anyone point me to a decent explanation or tutorial here at STM please?
If you're signing up to a network or direct advertiser to run their offers you'll sign up as an affiliate/publisher. You'd only sign up as an advertiser if you had your own products that you wanted to place on their network for other publsihers to run, or alternatively if you had exclusive campaigns from an advertiser and the network accepted third party offers you would also sign up as an advertiser.
It's actually quite simple, let me break it down for you 
Basically there are 3 entities in AM : publishers, advertiser and affiliates. Based on what you do, you could be classified as each of them 
Those who have traffic for sale are publishers, originally these were people with their own websites. So those selling traffic to the traffic networks are the original publishers. But since most of us here buy traffic, we become publishers too in a way too, because now we have some traffic for sale too.
Advertisers are people who are looking for traffic, usually they want to buy it. So the companies that own the offers that they need traffic for, these are the original advertisers. But again, since we buy traffic, we are becoming the advertisers in the eyes of traffic networks who sell us the traffic... we place ads (advertise) on the websites of their publishers.
Affiliate is someone who is promoting the offers/products of others, for a commission.
So let' break down a typical affiliate setup. Let's say I'm looking to promote a dating offer from a direct advertiser.
I sign up with the direct advertiser (offer owner) to get the offer from them, in this case I'm an affiliate or a publisher for the advertiser, because I'm gonna get them the traffic. Then I signup at a traffic network as an advertiser, because I am looking for traffic that I want to buy. And since I'm promoting the offer of someone else, I'm an affiliate.
To simplify it a bit... those who are buying traffic are advertisers, those who provide it are publishers (or affiliates, since they do that for a commission).
Does it make sense for you? 