On a biological level, free radicals are oxygen-containing moleculas that have an impaired electron. For example, superoxide ( O2-) and some other reactive oxygen species (e.g. OH-) are free radicals. Our cells produce thousands of free radicals every day through a process of oxidation in the body. Oxidation is an essential process in our bodies that generates energy, but  it is also damaging one because of free radicals reactivity. Since free radicals have at least one unpaired electron, they are unstable. In other words,  their atoms are not in a ground state. The presence of unpaired electrons make free radicals highly reactive. To become stabilised  or “ground” the free radical must obtain an electron from another molecular. When free radicals steal an electron from a surrounding molecule a new free radical is formed in its place. The newly formed radical then looks to return to its ground state by stealing electrons from cellular structures or molecules. Thus the chain reaction (of atoms stealing electrons) continues and can be thousands of events long.

Free radicals chip away at cell walls, molecule by molecule, making holes. The cells leak and lose their chemical balances. Subsequent free radicals are able to chip away at DNA, making cells dysfunctional. If this damage affects cellular DNA, the cell may malfunction. Such damage to DNA  often results in cancer and  are considered to be a primary driveforce in the aging process in the body.
Studies revealed that excessive amounts of free radicals in the body result in many diseases such as cancer, stroke, myocardial infarction, diabetes and major disorders. For example, free radicals can have destructive effects on LDL cholesterol (leading to heart disease) and proteins (reluting in cataracts).
The findings suggest that anywhere from 2 to 5% of the total oxygen intake during both rest and exercise have the ability to form the highly damaging superoxide radical via electron escape. During exercise oxygen consumption increases to 35-70 ml/kg/min. At the same time it was astimated that free radicals formation starts when we reach 3.5 ml/kg/min of the total oxygen intake during exercise.
It is ironic that oxygen (so indispensable for life), under certain circumstances, can also cause severely deleterious reactions in the human body which are primary driveforces in the aging process and can even result in cell death. To prevent free radical damage the body has a defense system of antioxidants.

Sources of Free Radicals:

Internally generated sources: Externally generated sources:
-oxidation during exercise
-xanthine oxidase
-reactions involving iron and -other transition metals
-arachidonate pathways
-ultraviolet light
-cigarette smoke
-environmental pollutants
-certain drugs, pesticides,
-anaesthetics and industrial solvents