Chemically speaking, antioxidants are molecules that prevent (hence, "anti") the oxidation (an unwanted reaction caused by exposure to oxygen, like the browning of an apple) of other molecules.
Antioxidants include vitamins A, C and E, minerals, carotenoids (organic plant-derived pigments) found in food, zinc, and a mineral known as selenium. Antioxidants are the body's primary defense against the effects of free radicals, protecting the body from cellular damage.
How much you should have: Antioxidants are measured in ORACs—the oxygen radical absorbance capacity. This method can be used to test the levels of antioxidants in spices, berries, legumes and other foods.
Many nutritionists recommend consuming about 5,000 ORAC units per day, though up to 30,000 may be beneficial. A serving of fruit (like one apple or tomato, or half a cup of strawberries) is about 500 units.