There's been a lot of speculation as Google begins facilitating image ads on sites, in addition to their text-based offerings. Folks ponder about the future of more traditional advertising networks like Advertising.com and Fastclick as Google moves into their space.
When discussing any advertising broker/aggregator, most folks first talk about the medium they do most business in. Google does most of its business in "paid search", and the second place medium is "text ads" on the "web". Now they are ramping up more traditional "banner ads" or "image ads" on the "web". Fastclick handles primarily "banner ads" on the "web". Azoogle handles primarily "email ads" on fairly legit email inventory.
So the dimension folks tend to focus on is the medium (web, email, search) and potentially the manifestation (text, image). Maybe even some details on the type of publishing partners they use (fairly legitimate, squeaky clean), or the type of targeting they do (contextual, advertiser specified, channel).
There are some neglected dimensions though, that are critical to understanding an advertising facilitator's business model.
-Number of clients
-Amount of integration (typically the greater the number of clients, the less the integration)
-Amount of risk the facilitator bears
-The method by which the advertiser is charged
-Primary targeting or matching methodology (how it is decided which advertiser is shown on which publishing location)
If you start to map all of this out against all of the networks, brokers, facilitators, aggregators, etc. it paints an amazing picture in regards to where the concentration is, where the growth is, and where in all honesty few companies have tread.....areas that show amazing promise that have largely gone overlooked.