A purchase order is a document that is sent from a buyer to a seller, specifying the type, quantity, and agreed-upon price for goods or services that the seller will provide to the buyer. An invoice is a document that is sent from a seller to a buyer, detailing the goods or services that the seller has provided to the buyer and the agreed-upon price for those goods or services.

A purchase order is typically the first step in the process of a buyer and seller doing business together, as it serves as a binding agreement between the two parties. An invoice is the final step in the process, as it serves as proof of payment for the goods or services that were provided. Together, a purchase order and invoice form the basis of a business transaction, allowing buyers and sellers to conduct business in a secure and organized manner.