Body Mass Index (BMI) is measure of body fat based on height and weight that applies to both adult men and women. BMI does not differentiate between body fat and muscle mass. Therefore, body builders and people who have a lot of muscle bulk will have a high BMI but are not overweight or obese.
Fat predominantly distributed around the waist is also a risk factor for health complications such as cardiovascular disease and type II diabetes. Fat predominantly deposited around the hips and buttocks doesn't have this same risk.