BMI, Body Mass Index, is a measure based on the ratio of a persons weight compared to their height. To calculate this you divide the individual's weight measured in kilograms by the square of the subject's height measured in meters: kg/m2.
The World Health Organisation (WHO) defines a BMI of over 25 as overweight and a BMI score of 30 or above as Obese. Whilst providing a useful rough estimate for both male and female and all ages, it should be used with some caution as it may not directly corresspond to the level of fat between different individuals. For example, a person with a high muscle mass may well have a high BMI score but that does not nessessarily mean they are fat.