http://en.wikipedia.org/wiki/Real-time_bidding
"Let us explain with a simple real time example, a user heads to a page on a publisher’s website, causing it to start loading. In the same instant the publisher sends out a “bid request” to thousands of potential advertisers saying, “We’ve got this user who is 30, Indian, male and based in New Jersey, US, and recently searched for return air tickets to Delhi, opening a page on our site. How much are you willing to bid for being the only ad on this page?” Within about 100 milliseconds the publisher gets bids from different advertisers, which then analyses to figure out the highest bidder and the brands being advertised. The winner is alerted by the publisher and allowed to place its ad on the page. The remarkable thing about this entire process is how fast and how often it takes place. The entire series of to-and-from communication between publisher and advertisers takes place in 300-500 milliseconds, causing no visible delay to the user. This process is repeated for every ad slot on a page."