I've been taking various fitness classes including BOSU. It's the name of the activity, as well as the name of the piece of equipment: an air-filled dome that is used like an aerobics step. BOSU is harder than step, because balancing on a squishy dome is more difficult. After all, balancing on a step is no different from standing on the floor.
The instructors I've had use the BOSU in various creative ways. You can do steps up and down, backwards or forwards squats, jump or hop up (and step down). You can turn it over, put your hands on it, and do "burpees" or run with your legs out behind you. You can lie down on it or sit against it and do an abs workout. You can combine it with weights or weighted medicine balls. Today we did a lot of high-cardio exercises on the BOSU. I could barely keep up.
It's fun to try new forms of exercise. The alternate class involves lying on the floor to stretch with my fellow little-old-ladies -- this is much more challenging, even if I flag from time to time. I hope that BOSU gets back to Michigan before I do.