Basically it allows you to specify the type of ad you want to be shown on a page, directory, or domain. JenSense has more information at http://www.jensense.com/archives/2005/09/new_ad_category.html and I posted some more information at http://www.seroundtable.com/archives/002508.html Keep in mind, my page should be showing "computer security" ads.
I know what you're doing... You get CPM ads and trick us into refreshing loads of times to get rich (Refreshed a dozen times or so, no Yahoo yet!)