healthy is the adj. of health, it means that you are in good health. e.g. She does lots of sports and keeps a balanced diet, so she is pretty healthy. healthful is also an adj. It means something is good for your health. e.g. Sports is healthful,it can make you healthy.

To add to what Grace has said, "healthy" is a commonly used word, and "healthful" is not. It is formal and practically obsolete.