From the scientific point of view oxidative stress is where oxidation exceeds the antioxidant systems because the balance between them has been lost. There is also a strong correlation between oxidative stress and lifestyle-related diseases (e.g. atherosclerosis, cancer, diabetes). Many daily habits are closely associated with oxidative stress, which is augmented by smoking, drinking and an irregular diet. It can cause serious damages to main constituents of the cells such as lipids, proteins or even DNA and thus influence the whole body.
To understand the nature of the oxidative stress it is necessary to know that usually atoms are composed of central nucleus with pairs of electrons orbiting around them. Sometimes however these atoms and molecules have unpaired electrons and are called free radicals. Because of those unpaired electrons free radicals are very unstable and highly reactive tending to form pairs with electrons “stolen” from other molecules. Oxygen may be one of those molecules, which is quite often converted to radical form in our organism. These molecules of oxygen are also called Reactive Oxygen Species or ROS. For the organism requiring oxygen for life, mechanisms that remove these highly reactive oxygen molecules are crucial, that is why during the evolution there were developed many antioxidant mechanisms controlling ROS.
Because of that dual role and many elaborate mechanisms that control free radicals they are not threatening the body in the physiological conditions. The balance between antioxidant and oxidative processes is however very fragile, and once free radicals are generated in excess or in abnormal sites that balance is lost and as a result oxidative stress arises. Then free radicals (and ROS) may attack molecules in cellular membranes and tissues leading to several kind of diseases (including heart diseases, cancer, pulmonary or neural diseases).
There are several ways to aid our internal antioxidant mechanisms. That is for example regular physical exercising or diet rich in antioxidants. Unfortunately antioxidants present in fruit or vegetables may have limited bioavailability. This is why supplementation proven effective is required.