Free radicals are chemical species possessing an unpaired electron.
Free radicals are very reactive,hence have very short life times.
They are over reactive because they are unbalanced.

Source of free radicals :
Drugs - Smoking - Radiation - Enviromental pollution

Summary :
Free radicals are byproducts of metabolism.
They are capable of causing cells to lose their structure,function and destroying them.
They are implicated in the development of 50 disease including heart, Diabetes and cancer.

How the oxidative stress can damage the body?