Carb blockers, also known as starch blockers,  are made up of bean and wheat germ extracts.  This mainly consists of white kidney beans, which are known to inhibit a digestive enzyme that breaks down starchy carbs.  Starchy carbs are food items like potatoes, bread, and pasta.

Carbohydrate blockers supposedly prevent carbs from being absorbed by the body.  Instead, the blockers make them pass through the digestive tract and be disposed over naturally.  Most pills available on the market today claim to inhibit about 30-45 grams of carbs from being absorbed.