This was something that I learnt in school but still struggle with because the wrong usage is so pervasive in what I read on a day to day basis. I often read articles saying, "this is a healthy diet" when it should be "this is a healthful diet".
The words healthy and healthful are closely related but they really mean different things. Healthy refers to something having good health. Hence, you can have a healthy body or a healthy business.
Healthful, however, refers to something that is conducive to health – in other words, salubrious (or health giving).
Thus, a diet cannot be healthy because a diet cannot have bad health. A person, on the other hand, can have good or bad health and hence a good diet is healthful, and it makes the person healthy.