Advertisers have been able to direct online messages based on demographics, income and even location, but one element has been largely missing until recently: immediacy. Advertisers booked slots in advance, and could not make on-the-fly decisions about what ads to show based on what people were doing on the Web.
Now, companies like Google, Yahoo and Microsoft let advertisers buy ads in the milliseconds between the time someone enters a site’s Web address and the moment the page appears. The technology, called real-time bidding, allows advertisers to examine site visitors one by one and bid to serve them ads almost instantly.
