Google AdWords Google AdWords is the world leader in pay-per-click advertising. Currently it has more than 150,000 advertisers. The ads show not only with Google search results, but also with Google's partners that include AOL search, About.com and thousands of other websites that publish AdWords ads. Google has an interesting ad ranking system. It ranks ads not by the bid (the amount their owners are ready to pay for one click), but by the combination of the bid and the click-through ratio.

This way, Google maximizes its revenue stream (since Revenue to Google = Bid x CTR x Views) and gives small advertisers an opportunity to effectively compete with big companies. A small advertiser cannot compete on the cost-per-click basis, but can successfully overcome any big company in terms of click-through ratio. AdWords ads can only contain 95 characters: 25 for the headline, then two 35-character-long description lines, and then a visible URL field. AdWords gives advertisers several options of keyword targeting: broad matching, exact matching, phrase matching, and negative keywords. The matching options define how close the search string entered by a user should be to a keyword selected by an advertiser.