Whats the difference

Depends...

Depends on what indicator you are using. If you are using BMI, then it's like this:

A BMI below 18.5 is characterized as underweight
A BMI of 18.5 - 24.999 is characterized as normal weight
A BMI of 25.0 - 29.999 is characterized as overweight or pre-obese
A BMI of 30.0 - 34.999 is characterized as obese
A BMI of 35.0 or higher is characterized as severely (or morbidly) obese

BMI isn't always an effective measure, though. Here's more:

http://en.wikipedia.org/wiki/Obesity