The difference is that fad diets are unsustainable from the start. Technically someone could try to continue a diet like this indefinitely if it had sufficient nutrients, but most people will get sick of a fad diet after a while and revert to their prior eating habits.

The difference between a fad diet and a good diet is that a good diet is designed to be sustainable from the start. The goal of a good diet is to reset your eating habits into a healthier set of foods that you can still enjoy and continue eating regularly indefinitely.