This explanation of the categorization and monetization of Internet search traffic at the Lightspeed Venture Partners Blog, is the most coherent and clear explanation I've read in a while. Excerpt below. Numbers based on Netscape.com data.
"In fact, around 20% of searches are “navigational” in nature - users looking for a particular website. Another 50% of searches are “informational” in nature (e.g. “capital of Taiwan”, “top social networks”) and the remaining 30% are “transactional” in nature (e.g. “cheap flights to Orlando”, “flat screen TV”.......It is relatively difficult to monetize navigational and informational searches. "