Obesity means having too much body fat. It is different from being overweight, which means weighing too much. The weight may come from muscle, bone, fat and/or body water. Both terms mean that a person's weight is greater than what's considered healthy for his or her height.

Obesity occurs over time when you eat more calories than you use. The balance between calories-in and calories-out differs for each person. Factors that might tip the balance include your genetic makeup, overeating, eating high-fat foods and not being physically active.