Online ad exchanges, which work a bit like a stock exchange to match buyers and sellers, are nothing new but they're starting to heat up again. And last month's launch of DoubleClick's Ad Exchange has helped continue to fuel the flames. But it will still be another year before the impact of ad exchanges will hit the mainstream. And at this point, the way that advertisers, agencies and publishers buy and sell advertising online will fundamentally shift.

An ad exchange is an open marketplace that facilitates the buying and selling of online advertising. It brings together buyers and sellers in an automated auction to achieve a fair price for online inventory - and then facilitates the placement of the buyer's creative on to the publisher's site. At its most basic level, an exchange addresses the underlying needs of buyers and sellers by providing the platform, community, controls and services.