Radicals are fragments of  molecules which are aggressive due to an unpaired electron.
 

Hexyl radicals (photo left) are fragments of hexane molecules which attack bromine molecules.
Bromine radicals (photo right) are fragments of bromine molecules which attack hexane molecules.

Chlorine radical are fragments of FCKW molecules which attack the ozone layer and desintegrate this gas.
Oxygen radicals attack the colour of black hair, the persons look "hydrogen peroxide blond".
Organic radicals are formed in our skin under hard UV radiation and may threaten our health.