difference between nil rated and exempted supplies also zero rated supplies

Nil rate of tax means that the law itself prescribes nil rate of duty for supply of such goods/services. Exempted goods/services are those where law prescribes rate of duty but the govt. has exempted by exective order called notification. The exemption can be partial or total. When the exemption withdrawn by notification again, the goods/services become chargeable to duty/tax again.