Things that are healthful are good for your body, mind, or overall well-being. Your dinner of barbecued potato chips, ice cream, and slices of cheddar cheese might be delicious, but it's not very healthful.
Something that has a favorable effect on your health can be described with the adjective healthful. Well-balanced meals full of whole grains and vegetables are healthful. A good night's sleep and a long walk in the fresh air are both healthful, because they promote your good health. When you are full of health, however, don't use healthful to describe yourself — instead, use the closely related word healthy.