Here's how the ads are targeted: Let's say you're reading about computers at tech site Engadget. An ad might appear offering special deals at Dell.com. Or, if you're reading an article at a news site about Attorney General Alberto Gonzales, you might see an ad for a local criminal-defense law firm.
"It was a really novel idea at the time to serve ads that were targeted dynamically" to a specific Web page, says Wojcicki, sitting in a conference room at the "Googleplex" company headquarters.
"People were saying, 'This is a sports site, so we'll serve a sports ad.' And we were saying, 'No. We can actually look at the page in real time and figure out what this page is about.' "
Wojcicki's idea turned into a runaway smash. Google doesn't break out revenues from AdSense and AdWords. But the company recently reported quarterly profit of $1 billion, virtually all derived from both ad programs.
Thousands of tiny entrepreneurs make substantial livings by hosting Ads by Google links at their websites. "More people make money from AdSense than any other vehicle on the Web," says Jennifer Slegg, who runs JenSense, a blog devoted to AdSense. "There are many, many AdSense millionaires."
AdSense "basically turned the Web into a giant Google billboard," says Danny Sullivan, editor of Search Engine Land website. "It effectively meant that Google could turn everyone's content into a place for Google ads."
For her efforts, Wojcicki earned a Google Founders' Award, a financial incentive provided to employees to create new ideas. Spokesman David Krane says it's designed to keep employees and give them the same kind of economic award they would receive if they had formed their own companies.
Krane won't disclose how much current Founders' Awards are worth, but the first two awarded to Googlers (not to Wojcicki) were $12 million each.
Creating things has kept Wojcicki at Google over the years. "I love taking an idea ... to a prototype and then to a product that millions of people use," she says.
"People write in from all over the world giving you feedback, telling us how a product (AdSense) changed their lives, how they were able to start a business with it, and that's just incredibly rewarding."
Her next big challenge: translating Google's simple, measurable advertising network to radio, TV and print.
Ever wanted to run an ad on the radio but didn't know how to set it up? Google has an Audio Ads section on its website with links to radio producers who will create an ad for $75 to $100. Select stations and time slots, and you're on the air.
The radio venture recently left test mode and is open to the general public. Google is testing the concept in print and on TV; that test is available only to a small percentage of advertisers.
Sullivan and other Google analysts are skeptical about the nascent program's chances. With radio ads, Sullivan says, Google "will give information on when the ad played, with more data than before, but that still doesn't prove that anyone heard the ad. They haven't done anything innovative or different to show if the ad really worked."