How do a fad diet and a healthy diet differ?

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.
Fads dont last. A fad diet is usually the diet of the moment. Many of them dont last. Some of them are extreme such as eating only grapefruit or watermelon, a healthy diet is one that takes into all aspects of nutrition, is well balanced, has a variety of foods involved and includes moderation in eating.