What is the difference between invoice and voucher?

Well, an invoice from a vendor is the bill that is received by the purchaser of goods or services from an outside supplier.
The vendor invoice lists the quantities of items, brief descriptions, prices, total amount due, credit terms, where to remit payment, etc.
A voucher is an internal document used in a company’s accounts payable department in order to collect and organize the necessary documentation and approvals before paying a vendor invoice.
The voucher acts as a cover page to which the following will be attached: vendor invoice, company’s purchase order, company’s receiving report, and other information needed to process the vendor invoice for payment.

A voucher it's internal document show what was the customer took form product or service before paid the cost of these and customer used it by give it to casher to paid the cost of these and take the invoice that show the cost of all the all product or service that customer purchased 

voucher it's internal document show what was the customer took form product or service before paid the cost of these and customer used it by give it to casher to paid the cost of these and take the invoice that show the cost of all the all product or service that customer purchased

In  a samole answer , Invoice is you receive and in order to receive it they are some procduers for accept it  But Voucher you release it according to your need for that mony value of this voucher and MUST be signed by the authorized person for payment , but taking in our answer that you mean Suppliers invoices and not SALES invocies as sales is also you release according to cycle of sales sections .

A voucher is a certificate or document proves that you paid for something

An invoice is a bill sent by a provider of a product or service to the purchaser.