What is the difference between a fad diet and a low calorie diet?

Diets: Fad diets are marketing ploys to promote a concept that has more to do with making money than with a well-balanced diet that promotes long-term compliance conducive with a healthy lifestyle. Many of these diets are also low calorie diets. A low calorie diet is a diet that provides extremely low calories in order to produce significant weight loss. They can be unbalanced and cause problems.

Depends: A fad diet is one that usually eliminates one or more essential foods or nutrients rom your diet: like the "cabbage soup" diet, and cannot be sustained very long. These diets are also over-hyped by companies/people looking to profit on your desperation to lose weight. A low calorie diet (adequate calories) can be great if proper nutrients are consumed.

Fad vs low cal: Most fad diets have a "gimmick" like- drink only water for three days, or eat only watermelon or grapefruit. In general there is a lot of questionable nutrition involved in these diets. They are for the most part calorie restricted and lead to weight loss by that method. A low calorie diet unlike many fad diets, can be a well balanced diet if you eat from all food groups protein, carb, fruits, veg.