The two major eating disorders are:
Anorexia nervosa, in which the person literally starves himself - she barely eats enough to survive, and even though she loses so much weight she looks like a skeleton, she still thinks she's fat.
Bulimimia nervosa, in which the person "binges and purges." She eats excessive amounts of food when she's nervous or anxious, and then forces herself to vomit afterward. Bulimia is dangerous because the person loses valuable nutrients and electrolytes by throwing up so often, and the stomach acid damages her esophagus and teeth over time.
The "war on obesity" results in many doctors telling their overweight patients that they must lose weight, it's a matter of life and death, and when a patient can't lose it via normal calorie-counting and exercise, they might resort to extreme measures and develop an eating disorder.