C Form is a Form issued by the sales tax department to the registered sales tax payer. If he buys any good from another registered sales tax payer and issues a c form to him. Than that other sales tax payer will not charge sales tax from him and sales tax will be collected and deposited by the c form issuer and deposited in the treasury of the govt. The other sales payer will deposit the c form to the govt. treasury. It means the issuer of the c form undertakes to pay the sales tax.
When you buy the goods outside your state you levied sales tax at cocensional rate and to get the credit the c form is issued by the seller and you can adjust your overall sales tax in the quarterly return.
It is a interstate sale where tax will be CST 2%, ie if ABC company suply material to XYZ Goa, 2% cst amount on the bill value and they XYZ will issue ABC C-form.