Over the course of 72 hours, they will be playing through as many of the games as they can in order to (hopefully) raise $800 for the charity. Games being played will range from the Classic series to Zero titles (and presumably X), 22 games in total.
In addition to the gameplay being streamed on Ustream.tv (which we'll try to provide an embed here for, if possible), there will be live commentary and a chat room in which the fans can interact with each other, as well as The Returners themselves. Donations will be handled by the ChipIn widget, "for ease of use and security." Anyone who has a PayPal account or active credit or debit card can donate, and those donations will go straight to the Alzheimer's Association.
You can also check out their official website (with Ustream.tv embedding) here.