The real dream of any advertiser is to grab the attention of the right person at the right time. A new approach to online advertising, known as real-time bidding, could help make that vision easier to achieve.

Real-time bidding involves auctioning off the opportunity to show an online display advertisement to a specific type of user at a precise moment. A San Francisco-based startup called Triggit recently scored $4.2 million in funding from two venture capital firms, Foundry Group and Spark Capital, based on the promise of its real-time bidding platform.

"Every person has a different value to different advertisers," says Zach Coelius, Triggit's CEO. He points to the advertising auction system used by Google for search keyword. People searching for particular keywords are bracketed together as likely having similar intentions. With display advertising, he says, the interests of the person visiting a page is less clear, and it's more difficult to match an ad to the ideal user. It is increasingly possible to gather information about a user by looking at her browser's cookies--tiny files that show which sites she has visited. But matching this information to advertising is a still relatively crude process.

Real-time bidding lets advertisers bid against each other to show advertising users based on different pieces of information about that person and their behavior. Bids and counter-bids are made in the microseconds before the winning ad is served up on a page.

Triggit's technology processes about 15 billion Web impressions a day. The company works with 10 companies that provide data on Web users' demographics and interests based on tracking the sites they visit online, and offers inventory from nine online ad exchanges.

When setting the price for a particular advertising opportunity, Triggit considers the user's browsing history, the type of site currently being visited, and other details. The company then works out how much money the ads on a particular website are worth, and sets a price. An ad is automatically auctioned quickly enough to serve the ad to the user without perceived delay.