The band itself is nicely designed; it's just a little device that straps around your upper arm comfortably. It's designed to be worn as much as possible -- Stivoric said that some users just wear it during workouts, but many wear it more often than that. It's got a three-axis accelerometer on board, and it has equipment for monitoring your body's skin temperature and conductivity, as well as a few other measures of health and activity.
The "BW" in the product's name marks the fact that it's Bluetooth-enabled. That means it can hook directly up to the company's free app, which it will do after being paired whenever the two are in proximity. What you get is a solid record of your body's activity throughout the day, and by inputting your calorie intake and even sleep records into the app, BodyMedia can help you work steadily toward your own fitness goals.