Harvard School of Public Health: Carbohydrates - Explains what carbohydrates are, the difference between high-glycemic and low-glycemic carbs, and the benefits and risks of low-carb diets.
Wikipedia: Carbohydrate - Explains the structures of different types of carbohydrates, and the role they play in nutrition.
Carbohydrates in Nutrition - Article on the complex and simple types of carbohydrates and the result of excess carbohydrate intake.