You switch to healthier carbohydrates (high fiber, low glycemic index) that don't spike your blood sugar. Also, you're encouraged to eat healthier fats and lean sources of protein.

There are three phases to South Beach. Phase one is similar to Atkins where you eliminate most carbs for two weeks. This jump starts your weight loss.

In phase two, you add back in healthy carbs like whole grains and fruits. You stay in phase two until you reach your goal weight.

Phase three is long term maintenance. You just have to stick to eating the right kinds of carbs, fats, and proteins are you can maintain your weight.

From the back-cover of "The South Beach Diet":

For years cardiologist Arthur Agatston, M.D., urged his patients to lose weight for the sake of their hearts, but every diet was too hard to follow or its restrictions were too harsh. Some were downright dangerous. Nobody seemed to be able to stick with low-fat regimens for any length of time. And a diet is useless if you can't stick with it.
So Dr. Agatston developed his own. The South Beach Diet isn't complicated, and it doesn't require that you go hungry. You'll enjoy normal-size helpings of meat, poultry, and fish. You'll also eat eggs, cheese, nuts, and vegetables. Snacks are required. You'll learn to avoid the bad carbs, like white flour, white sugar, and baked potatoes. Best of all, as you lose weight, you'll lose that stubborn belly fat first!
Dr. Agaston's diet has produced consistently dramatic results (8 to 13 pounds lose in the first 2 weeks!) and has become a media sensation in South Florida. Now you, too, can join the ranks of the fit and fabulous with The South Beach Diet.