From NuGOwiki

Phytonutrients are also referred to as phytochemicals. Phytonutrients naturally occur in vegetables and fruit. In broad terms, they are said to be any chemical or nutrient derived from a plant source. However, in common usage, they have a more limited definition. They are usually used to refer to compounds found in plants that are not required for normal functioning of the body but that nonetheless have a beneficial effect on health or an active role in the amelioration of disease. Thus, they differ from what are traditionally termed nutrients in that they are not a necessity for normal metabolism, and their absence will not result in a deficiency disease. Besides flavonoids, the following families of phytochemicals are known:

• isoflavones
• isothiocyanates
• monoterpenes
• organosulfur compounds
• saponins
• capsaicinoids
• phytosterols

More information on phytonutrients can be found on the Wikipedia link.