What is the difference between an invoice and a voucher?

Invoice if received is an external document,if issued also is intended for external use. Invoice whether received or issued is a legal claim of the cost of the goods or services both ways.Invoice requires authentication of the concerned authority for issuance.

Voucher is an internal document.Voucher requires the approval of the concerned authority for payment.Voucher can be based on an invoice, whereas the vice versa is not normally there.

An invoice in layman's term is a bill. Ie: cellphone bill or electric bill while a voucher can be a gift cheque that you can use for a free product/service. Others use the term voucher for receipts.

invoice contain amount and quantity,  voucher contains only amounts

All kind of documentary evidence of any financial transaction is called voucher & invoice specifically pertain to seller and purchaser. And invoice is also a voucher because it is a documentary proof of financial transaction.

Invoice is the bill given to customer for payment whereas voucher is document authenticating the entry of the bill into accounting records. 

1- Invoice or bill: is a detailed list of items you bought or sold includes quantities, prices and values ... etc

2- Voucher: is a formal document of items you bought or sold, could be detailed or not (usually signed and sealed with tax ID) with total value and description to insure valid transaction.

Invoice is made for the particular purpose of sale [sales invoice] or in come cases for purchases especially in software entries. 

Voucher on the other hand is for any transaction. 

An invoice is either received from a supplier (for purchases or services) or issued to a customer (for sales or services). A voucher is an internal document...can be a payment voucher, receipt voucher or a journal voucher.

As per my knowledge,  invoce is the document get from the external source or supplier to the company

where as vouchers are the documents used by company to pay the dues and bills to the other parties.

In hotels, the invoice is the final billing with total of charges and payments, but the voucher is not an invoice is like a purchase order sent from the agent when the customer come through the agent and in the end of customer stay we can print a final invoice for the agent.