An ad exchange is an online marketplace where advertisers and publishers can buy and sell ad space in real time. Video ads, display ads, and mobile ad inventory are the most popular ones to be sold on Ad Exchanges. 

Advertisers either use their own bidding technologies or demand side platforms (DSP) to buy from an ad exchange. Most Ad Exchanges like the Rubicon Project, AppNexus, and OpenX can function programmatically to allow advertisers and agencies to automate their buyings. 

There are two different kinds of ad exchanges. open exchanges or private exchanges. While open exchanges are open to everyone, the private exchanges are for the publishers who want to know who to sell and at what price.