An advertising network (also known as “ad network” or “banner network”) is a group of websites connected by a sort of agency that allows them to use its online platform for ads exchange.
In other words – advertising network contains two groups: advertisers and publishers (while it should be noted that one can be a publisher and an advertiser at the same).

The forms of advertisements that could be exchanged through an “advertising network” are varied and include, among other, banners (or image ads), textual ads, landing pages and video advertisements.

The main types of online advertising networks are blind networks, targeted networks and representative networks.
Being a part of a Blind Network enables you to advertise your service or product for a low price, but with no control over location (you can’t choose where your ads will appear).
In a Targeted Network the advertiser’s ads appear in a highly relevant websites (this is why it is called a targeted network) from the same niche and context.
A Representative Network offers a revenue share model (payment = a certain percentage of sales revenues generated by clients who were referred via the advertiser’s ads). Contrary to the “blind network”, the representative network is completely transparent in regards to letting the advertiser know where his advertisements will run.

Other terms relevant to the field of online advertising network are first-tier and second-tier networks.
A first tier advertising network is a big network of advertisers and publishers (all are part of the network) with high quality traffic, which is usually used by second-tier networks.
The “big 3” (Yahoo! Google and MSN / Bing search engines) is an excellent example of first-tier networks

A second-tier advertising network is, comparing to the first-tier network,  a smaller network in which the main source of income comes from syndicating ads from other advertising networks.