An invoice or bill is a commercial document issued by the seller to the buyer indicating the products, quantities and agreed prices for products or services the seller has provided the buyer. An invoice indicates the buyer must pay the seller according to the payment terms. The buyer has a maximum number of days to pay for these goods and is some times offered a discount if paid before the due date.
An invoice is a statement of what you owe for the item(s) and usually terms of payment.
A voucher is a bond which is worth a certain monetary value and which may be spent only for specific reasons or on specific goods.
It is a document recording a liability or allowing for the payment of a liability.