VAT is Value Added Tax. It is tax chared by the registered
dealer at the time of sale of goods just like Sales Tax. A
vat dealer can claim credit of vat on good purchased by him
just like excise duty.
While Excise is the tax on goods produced but charged at the
time of its removal i.e. at the time of sale.
Form C is issued by a reg. dealer(purchaser) to seller of
goods in case of CST so as to charge him sales tax at lesser
rate whereas the form D isissued by the govt to the seller
of goods for the same reason.