Web inventory is not all evenly valued. At the top of the funnel is inventory that has high demand. Bennett Zucker, formerly of Tacoda and now of Right Media, estimates that as little as 25 to 30 percent of all inventory is "highly desirable." This includes both premium placements on portal home pages or channels of portals sold on a daily basis and categories like auto, financial services (on a year-round basis, but with exceptional pressure during the tax season), health areas (such as diabetes and cholesterol), retail mall, movie listings and anything where real time information is needed.

In addition, contextual placements are generally accepted to be the most effective. The common perception among most clients -- and many agencies -- is that ad placements that are contextually appropriate within related edit are most effective. More about the uniqueness of the auto industry, the web's biggest advertising category below.

As inventory that is in greatest demand is sold, whether it be for branding or direct response (DR), a different marketplace takes over. It is one of targeting, auctions and networks. The targeting can take the form of traditional demographic or psychographic (sociographic or lifestyle) or behavioral targeting.