This is the "Jawbone UP wristband." It basically monitors your sleep and your movement throughout the day, I'm not entirely sure how it does it, but it does. It connects to my phone where my headphones would normally go and sync's with the Jawbone UP app which is free.
You can set goals for the amount of steps you would like to do in a day, goals for sleep if you sleep too much or not enough and goals for weight. Now personally for me I wanted this not for losing weight as I am perfectly happy with my weight and so are my doctors, I wanted this because I need to try and maintain some sort of an activity level. Now we all know I'm not going to be doing 10,000 steps a day or jogging or running but I would like to just be that little bit more active. By doing so this will hopefully keep me be "fit enough" for when I get my transplant. I don't want to be completely incapable of doing anything when it finally decides to happen.