Advertisers though, able to post messages online – based on income, demographics and even location – have been missing out on immediacy factor thus far. In other words, required to book slots in advance, they largely fail to take on-the-fly choices about which ads to display based on what users were doing on the Web.

Now Google, Microsoft and Yahoo let them buy ads in those split milliseconds – between the fleeting moment when a user punches a site’s Web address and the fraction of time in which the page actually appears. Termed real-time bidding, the technology lets advertisers examine visitors one-by-one and accordingly bid to serve ads to them instantly.

Practically speaking, say a user has just searched for leading golf club brands on eBay (It’s testing a system from a firm AppNexus for over a year). It can essentially track the user’s activities almost in real time, thus deciding where and when to display near-personalized ads for the relevant product (golf clubs, in this case) throughout the Web.

If eBay notes that the person got a driver at some other website, it can immediately update the ad to begin showing tees, balls or perhaps a package vacation to a popular golfing destination. If a woman customer is shopping, it could modify the ad’s color or the entire presentation.

According to marketing experts, it makes sense to arrive at such key decisions as close to when the advertisements run, as possible. This helps overcome the problem with online advertising that the choice to show them something is made much in advance of when they are actually displayed to the users.

Compare real-time bidding to billboard ad. In the former, billboard space gets auctioned off literally second by second. It’s tailored to each viewer. The vice president (product management) of Google, Neal Mohan, claims: “Information can change dramatically even throughout the course of a day. The more precise you can be in terms of being able to act on it as soon as you get that information, the better it will be.”