Healthful describes something that will create good health, like apples, yoga, and fresh air. Healthy describes someone fit, trim, and utterly not sick.

Healthful food and exercise fills you full of health! Healthful has meant something that will create good health since 1398, and that remains the main definition of the word:

Choosing the most healthful foods for your family is no easy task. (Time) Tilapia crusted in pulverized plantain chips, with sweet maduros on the side, was a relatively healthful departure. (New York Times)

Healthy is used to describe someone or something that enjoys all that good health. If you're healthy, whether you're a tree, a bank account, or a human, you're well, thank you very much. Here are some healthy tidbits from the news:

Any team interested in Sizemore will need to be convinced he is, finally, healthy again. (Washington Post) Healthy forests are built to withstand severe natural disturbances. (New York Times)

This word pair, healthful and healthy, has been causing debate for over a century. The question is whether these adjectives can both be used to mean conducive to good health. This is what gets some word mavens' blood boiling. Healthy, they say, cannot be used to mean conducive to good health. But according to the Oxford English Dictionary, healthy has been a synonym for healthful since its earliest appearance in print... in 1552.

So it's OK to use healthy and healthful as synonyms for conducive to good health: have a healthy snack or a healthful one. But if you're referring to someone who enjoys good health, however, use healthy because it'd be weird to call a person healthful. Save healthful for the granola and healthy for your personal trainer.