Simple sugars differ from complex carbohydrates primarily in their chemical structure. Simple sugars, or monosaccharides, consist of single sugar molecules, like glucose and fructose, and are quickly absorbed into the bloodstream, leading to rapid energy spikes. In contrast, complex carbohydrates contain multiple sugar molecules and take longer to digest, providing sustained energy. Foods rich in complex carbohydrates, such as whole grains and legumes, are generally more nutritious than those high in simple sugars, like candies and sodas, as they offer essential fiber, vitamins, and minerals.