Here are the basics: this afternoon, when Big Picture goes live, you'll be able to push a button and turn Steam into an entirely new interface. It sort of looks like the dashboard on an Xbox 360, minus the advertisements and other clutter that can make that system so irritating to navigate. And it allows you to do almost everything you can do on vanilla Steam: you can buy games, browse the web, and even chat with your friends using the platform's standard in-game overlay.
Check out the post which is brimming with screenshots and more info.