Americans include fiber, sugar alcohol and other elements as part of total carbohydrates. By net carbs, they mean: total carbs minus fiber and sugar alcohol, if any. Since their labels are required to present the carbs this way, total carbs are overstated if you don't subtract fiber.
Now, there is also some controversy whether sugar alcohol should be subtracted to arrive at net carbs. Many people agree that fiber should, however. Most diet books (Atkins, etc.), when they refer to 30 carbs per day, for example, means 30 carbs net of fiber. So if you eat a lot of fiber, like 30g, you can eat up to 60g total carbs and stay at your 30 carbs per day diet.