Body composition refers to the ratio of fat to lean mass in the body, while BMI (Body Mass Index) is a measure of weight relative to height. BMI does not differentiate between fat and muscle, leading to potential misclassifications, as individuals with high muscle mass may appear overweight. In contrast, body composition offers a clearer picture of health by indicating how much body fat one has. This differentiation is crucial for tailoring fitness and nutrition plans to achieve optimal health and fitness outcomes.