(FORTUNE Magazine) – NEW YORK CITY Founded 1996 Revs.: $10 million (est.) Employees: 25 Private www.doubleclick.net

You sign on to your favorite Website and voila!--up pops an ad for Happy Times Cruise Lines. "Escape the Northeast winter blues with Happy Times," beckons the ad. "Click here to find out about our exciting executive promotion." Sure enough, you work in Connecticut, and you've been thinking about vacationing in the Mediterranean. But how do they know that? Whoever they are.

The omniscient "they" is probably DoubleClick, an advertising broker that serves both companies with Websites and advertisers looking to reach Webheads. It currently works with 25 sites, including those of USA Today, Intuit, and General Electric. DoubleClick has inked deals with more than 35 advertisers, like IBM, BankAmerica, and Nissan.

How does it work? Let's say that the last time you went online you clicked on pages about travel, cypress trees, and Italian wines. Those pages, which happen to sit on Double-Click sites, are quickly downloaded on to your PC. DoubleClick's software notes that those packets of data went to your Internet address. Just like that, DoubleClick starts to build a profile of you and your interests. Since starting in March, says CEO Kevin O'Connor, 35, "we've identified four million individuals, and we're adding somewhere around 100,000 every day."

O'Connor also uses the data to create lists of user categories--gardening, travel, sports, and so on--that advertisers "buy." DoubleClick sells ad banners on member sites, taking a 30% to 50% cut of revenues.

So chances are that during the last online search for the perfect vacation, you gave DoubleClick enough clues to guess that you might be interested in a Mediterranean cruise. The next time you log on to a Double-Click site, its software notes your E-mail address, checks out your user profile, and uploads an ad customized for you--within milliseconds of your signing on.

DoubleClick was formed by the merger of a software startup and a division of ad agency Poppe Tyson--uniting nerd culture and the slick savvy of Madison Avenue. "The two cultures are completely ignorant of each other's ways," O'Connor says. "Our one common bond is love of the Internet."

--Melanie Warner