On the other hand, Obesity is the condition of being obese, While an overweight person will carry excess weight, he may or may not have excess accumulation of fat. Sourced from: http://www.diffen.com/difference/Obesity_vs_Overweight

Overweight and obese persons have excess amounts of weight or fat. The most common method of separation between overweight and obese persons is by the use of the Body Mass Index (BMI).

BMI can be used as a screening tool but is not diagnostic of the body fatness or health of an individual. If your BMI is less than 18.5, it falls within the underweight range. If your BMI is 18.5 to 24.9, it falls within the normal or healthy weight range. If your BMI is 25.0 to 29.9, it falls within the overweight range. If your BMI is 30.0 or higher, it falls within the obese range. Sourced from: http://www.cdc.gov/obesity/adult/defining.html

Stage 0: You do not have any health problems related to your weight. Stage 1: Any weight-related health problems are mild (such as borderline high blood pressure or occasional aches and pains). Stage 2: You have an obesity-related chronic disease, such as high blood pressure, type 2 diabetes, sleep apnea, or osteoarthritis, and you have moderate problems doing daily activities or feeling well. Stage 3: You have had serious weight-related problems, such as a heart attack, heart failure, stroke, or other conditions. Stage 4: This is the most severe level of weight-related chronic health conditions, which are extreme and life threatening. Sourced from: http://www.webmd.com/diet/am-i-obese?page=2