Valve released a trailer giving a few details of Big Picture Mode's beta.
At first glimpse, it seems that Steam's Big Picture Mode, which can be accessed with a simple click on an appropriately labeled transparent bubble, simplifies Steam's interface to be tile-like (think Xbox Live) to be more easily navigable with a controller.
Big Picture Mode will retain all of Steam's features, including the browser, which will feature tabbed browsing and cloud-saved favorites. Typing with Big Picture Mode has—thankfully—been simplified. Shunning the QWERTY layout, typing on Steam in Big Picture Mode involves the Daisywheel, where four letters are assigned to the four face buttons. Every letter is grouped into four and displayed in a wheel, and each grouping can be cycled between using the d-pad or analog sticks.
For more details, go to Steam's official page for Big Picture Mode.