The principle behind a low-calorie diet is simple.  Since calories provide our bodies with fuel, if we reduce the number of calories consumed, the body will need to burn fat in order to produce necessary energy.  Of course, too much of a good thing, even weight loss, can be bad.  Lowering the calorie intake too much can lead to a number of consequences.     

While a regular diet typically includes between 2000 and 2500 calories a day, some very low calorie diets can require participants to consume less than 800 calories.  Such a practice places a great strain on the body and can be extremely dangerous.  Following such a diet for more than a day or two can play havoc with your entire system.  Potential side effects include fatigue, constipation, diarrhea, and nausea.
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Losing weight should be done gradually over time.  Rushing into a low-calorie diet with absurd restrictions will no doubt produce immediate and dramatic results, but it's hardly the healthiest way to achieve your weight loss goals.  Low-calorie diets also tend to trigger a, "yo yo," effect.  As soon as you go off the diet, all the weight you lost will return, pushing your weight to its previous level if not higher.  When you're on the diet, your body's metabolism slows down in order to compensate for the lack of calories.  Once you start eating normally again, your metabolism doesn't speed up, making it easier to pack on the pounds.