There are various eating disorders and you may have heard of some of them.  Anorexia and Bulimia are ones that are commonly spoken about.  Both are driven by disordered thoughts about weight, shape and control over eating which result (in the case of Anorexia) in severe restricting of energy into the body (compared with what the body actually needs) such that a person becomes very underweight.  In the case of Bulimia, while the person restricts their food intake, they find themselves on occasion bingeing or eating more than they are happy with and then compensating for this in some way (e.g. via vomiting, using laxatives, diet pills, over-exercising or diabetic people omitting their insulin).  People with Anorexia can use these means of compensating for eating too but the main difference might be that their weight has plummeted to a lower level.