BDD, Body Dysmorphic Disorder is when a person believes they look different than they really do. They might believe their nose is gigantic when in fact it is a normal size. They might believe their thighs are unusually large when they are normal or even under developed because the person is underweight.
Body Dysmorphic Disorder is not an eating disorder. However, it shares many of the same characteristics and thought processes as someone with anorexia or bulimia. You cannot be diagnosed with BDD if the hatred is for your body shape, size and/or weight. That is accounted for with the diagnosis of Anorexia Nervosa. But if the preoccupation is with one particular body part, concerning its size, shape, the amount of fat on it, then BDD is probably an appropriate diagnosis.
The view from the outside world
As you find in eating disordered persons, someone with Body Dysmorphic Disorder has an unrealistic view of themselves. They cannot be convinced that their nose is not crooked, that their eyes are wide enough apart, that their stomach is flat enough. As a result of their extreme preoccupation with their looks, someone with BDD is bound to isolate themselves from the rest of the world. That, or they will attempt to use plastic surgery to "correct" their "problem."
Someone with BDD believes that the rest of the world stares at their hated part and notices the imperfections as much as they do. The reality is that very few people would agree that the hated part is how the person with BDD describes it. It's just not a realistic view.
We're all dissatisfied with some aspect of our appearance
It is normal to dislike some aspect of your body. Maybe you feel like your chin isn't prominent enough. Maybe your nose is a little too big or a little too crooked. Maybe your lips are too full or not full enough. Maybe your breasts are tiny ... or too large for your frame. I could go on and on.
There's nothing wrong with disliking parts of your body. There's even nothing wrong with doing something to correct your disliked part. I had a breast reduction because I couldn't stand how large my breasts were. That doesn't mean I have BDD.
The difference is in the obsession. With body dysmorphic disorder, it is nearly impossible for the person to stop concentrating on the hated body part - to the point of being obsessed. Some people will even resort to their own means to try to correct the problem - even to the point of attempting to cut off limbs.
Self Esteem
A similar characteristic of eating disordered people, those with BDD have low self esteem. This is not a cause of BDD, but it certainly contributes to the development.
Diagnostic Criteria
According to the DSM (Diagnostic and Statistical Manual of Mental Disorders), there are specific criteria for Body Dysmorphic Disorder. They are:
Preoccupation with an imagined defect in appearance. If a slight physical anomaly is present, the person's concern is markedly excessive.
The preoccupation causes clinically significant distress or impairment in social, occupational, or other important areas of functioning.
The preoccupation is not better accounted for by another mental disorder (e.g., dissatisfaction with body shape and size in Anorexia Nervosa).