>> BTW, buying is not a huge part of adwords. In fact, it's pretty hard to get a positive ROI on a "product" purchase campaign.

Great point.

If I understand you correctly you are talking about how adwords link to marketing landing pages that _give away_ something valuable to the user like an ebook in exchange for your email address. In this way they convert you by slowly giving away more value, building trust, and upselling premium offerings, etc.