My sister and my parents both have one.
You get a balance board which has pressure pads that sense how far left/right/forwards/backwards you lean and you use this to control various "games".
For example, there is a ski-jump where you bend your legs and lean forward to get up speed, then straighten your legs quickly to jump. There is a slalom so you lean left and right to ski through markers.
Then there are Yoga moves - you copy the character on-screen and try to maintain you centre of gravity which can be a lot more painful and difficult than it sounds!
Aerobic exercises include things like hoopla and step aerobics. 2-player jogging is hilarious!
It's very good and quite easy to work up a sweat with it!!