Big Picture is the new user-interface Valve's targeting as a full-screen mode for running Steam on your television set and other devices with very large displays. This is also leading to their eventual foray as a Linux-based gaming hardware console.
Big Picture also includes their own web browser from the Steam client. Steam Big Picture is optimally controlled from a traditional game controller while keyboard and mouse input is also supported.
Below is a video that Valve produced to introduce this new Big Picture mode currently in testing. Big Picture will work on platforms where Steam is supported, including Windows, OS X, and the forthcoming native Linux client.
There's also a profile of Valve worth reading over at the New York Times.