Google and Yahoo currently allow only limited targeting of search ads by location.

A less obvious driver of search ads, but perhaps as important in the long run, is that they sell themselves.

So they are both placing search ads on other Web pages.

Google did not invent the concept of keyword search ads, which predates the company's founding in 1998.

To find additional traffic for these advertisers, Google began showcasing search ads on third-party sites across the web.

No one outside of the discussions knows for sure, but speculation centers around search ads.

The businesses stop short of handing cash to users who may be tempted to sit at home clicking search ads between naps.

Ad revenue came from banners, contextual search ads, and paid placement of featured video sites.

Large established companies, which are mostly interested in furthering their brand recognition online, buy far more display advertisements across the Internet than search ads.

The company has said that its overall online advertising revenue, which includes sources beyond search ads, reached $1 billion in the past year.