In this sense, radicals are often simply referred to as groups, a functional group being one responsible for the characteristic reactions of a class of compounds. Thus, the hydroxyl group is the functional group of alcohols.
Free radicals are groups existing independently.
Because they have unpaired electrons they are usually extremely reactive transient species. They can be produced by pyrolysis or photolysis and are important intermediates in many reactions, particularly photochemical, combustion, and polymerization reactions. Oxygen free radicals are formed within the body during normal metabolic processes and as a response to infection and toxins. They can cause cell damage, which may be neutralized or prevented by antioxidant nutrients. The Macmillan Encyclopedia 2001 2.