Basically, it is calorie-intake based, promotes many small meals, lots of fruits & veggies and protein, eating only whole grains while cutting out all "appetite stimulating" white foods (breads, pasta, potatoes). It allows for a cheat-food (sugars, alcohol, etc.) of up to 200 calories per day too. The exercise plan isn't really laid out in detail on this site, but it looks like it includes an hour of exercise per day (once you work up to that).
As I was reading that I realized that I don't think that anyone in our Biggest Loser contest is actually doing the Biggest Loser diet... and I thought that was kinda ironic. :)