According to Wikipedia:
Phytonutrients are nutrients derived from plant material that have been shown to be necessary for sustaining human life. Phytochemicals are non-nutritive plant chemicals that contain protective, disease-preventing, compounds. Their role in plants is to protect plants from disease, injuries, insects, drought, excessive heat, ultraviolet rays, and poisons or pollutants in the air or soil. They form part of the plants immune system.
Although phytochemicals are not yet classified as nutrients, substances necessary for sustaining life, they have been identified as containing properties for aiding in disease prevention. Phytochemicals are associated with the prevention and/or treatment of at least four of the leading causes of death in Western countries - cancer, diabetes, cardiovascular disease, and hypertension. They are involved in many processes including ones that help prevent cell damage, prevent cancer cell replication, and decrease cholesterol levels.
So, the next time you're reaching for a snack, consider that the fruit or vegetable you're grabbing (right? :) is super healthy and healing.