Simply click either button to post the track or the entire station to your Twitter feed or Facebook Wall. The first time you use either button you'll be prompted to connect with the relevant service via OAuth or Facebook Connect, respectively.
As Pandora says on its blog, this feature completes the triumvirate of discover, buy, and share we tend to want to do with the music we love. Being one-click away from sharing what we're listening to definitely adds to the Pandora experience. However, the buttons haven't yet made their way into the Pandora One desktop app, which would be great to see eventually.
With this sharing focused update, the "gift a station to a friend" feature has also been moved to the foreground -- something to keep in your back pocket as a unique and low-cost gift idea for your music-loving friends and family.