what is radio activity.what it is differ from normal substance

dear nithin 
radioactivity is a spontaneous emission of energy from an unstable atoms , there are stable atoms , which remain same always but unstable which break down or decay into new atoms . 
these unstable atoms are called radiactive beacuse they emit radioactivity from the nucleus as they decay . 
and it do varies from normal substances as normal substances are stable atoms , but those atoms which are unstable undergo into radioactive emission . 
HOPE IT CLEARS 
REGARDS