The Body Mass Index or BMI is measure of body fat based on height & weight that applies to both adult men & women.
BMI does not differentiate between body fat & muscle mass. So, body builders & people who have a lot of muscle bulk will have a high BMI but will not be classified as overweight or obese.
Different Types of Fat
An obese person can develop a certain body profile. There are Two different types of fat deposits. Fat predominantly distributed around the waist is also a risk factor for health complications such as cardiovascular disease & type II diabetes. Fat predominantly deposited around the hips & buttocks does not seem to carry this same risk level.
References Victorian Government, Better Health Channel, available at http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Body_Mass_Index_(BMI)?opendocument, last accessed 09/01/11 World Health Organisation, Global Database on Body Mass Index, available at http://apps.who.int/bmi/index.jsp?introPage=intro_3.html , last accessed 09/01/11