The Big Picture mode will feature a version of Steam that can be navigated with a gamepad and will be viewable with larger fronts and graphics that are more suitable for your HDTV. Of course, while you can opt to control Steam in the Big Picture mode with your gamepad, you can also use a mouse and a keyboard, if you like.
Despite all of the rumors, Valve insists that the decision to deliver the Big Picture has nothing to do with the possibility of a Valve-style PC console all-in-one that would run Steam. Sources are telling us a different story, however, and suggest that Valve will be measuring the response to the Big Picture mode closely; and if it proves popular, who knows what Valve might offer next.