Free Radicals are cellular bad guys. They are rogue molecules that seek out other molecules (cells) they can “steal” electrons from. If free radicals are not quickly destroyed they can cause a lot of damage and set the stage for degeneration. They can attack and oxidize the DNA of healthy cells and they can attack healthy cell membranes.
Oxygen-containing molecules that turn into oxidized agents are called reactive oxygen species (ROS). The danger of these oxidized agents is that they have lost an electron and, like free radicals, they roam around looking for other electrons they can steal. Dr. Denham Harman (father of the Free Radical Theory of Aging) hypothesized that oxygen radicals are responsible for gross cellular damage, genetic mutations, and the degenerative process of biological aging 
Oxidative stress occurs when there is:
- An overproduction/overabundance of reactive oxygen species/free radicals
- A depletion of the antioxidant defenses
Translation: Too many bad guys and not enough good guys!
According to leading experts, disease and degeneration often occurs because the body is in a state of oxidative stress. This is why a diet rich in antioxidants is so vitally important.
Antioxidants are the body’s defense mechanism that prevents free radicals, reactive oxygen species and/or the oxidative stress from damaging or killing healthy cells. In short, these compounds gobble up reactive oxygen species and free radicals and chemically neutralize them.
If you cut an apple and leave it out in the open air it will begin to oxidize and turn brown. This is oxidative stress. However, if you pour orange juice on it after you cut it, the ascorbic acid (vitamin C) will protect the apple against oxidation and prevent it from turning brown.
Hundreds of powerful antioxidants are found in plant life (i.e., vegetables, fruits, grasses etc.). Examples of antioxidants include: Vitamin C, Vitamin E, Zinc, Selenium, CoQ10, Glutathione, Alpha Lipoic Acid, NAC, Turmeric, gallocatechin and many, many, many more. The best defense is a strong offense, so make sure your body and immune system have the advantage.
 Droge, W. “Free radicals in the Physiological Control of Cell Function.” Physiological Review 82 (2002):47-95.