Okay I have a few questions about ad relevancy: First of all I am wondering if the ads displayed on index.html are the same throughout the entire page no matter what the content is (specially useful if running a article page)? Is the results based on frequency of a keyword found in html (ie. most used keyword in the page wins)? or does it follow some other factors? Thats it to start!
Also, are ads slightly relevant to thew viewer as well, based on geographic location/last seen pages/cookies etc.?
Each page gets its own set of ads, but yes, every site does get a "general category ads" (i.e. if you have a car site, when you ad new pages, your new pages would get generic car ads until Adsense gets a chance to spider those new pages and delivers relevant ads to the content). And factors include frequency of keywords and ad availability (i.e. does Adsense have ads to serve you). Some ads uses geo-targeting (dating ads, city-specific ads) and others are general industry ads.