A purchase order is issued by the buyer to the seller indicating the type of products purchased, quantity of the product, the price and the total amount the buyer is willing to pay the seller. A purchase order constitutes as a legal binding document that a buyer send to the seller.
An invoice is bill that is issued by the seller to the buyer, stating the products, quantities, agreed prices of the product or service that is being provided to the buyer. An invoice states the sale transactions only. Payment terms are also mentioned on the invoice stating how and when the buyer is expected to pay or if the buyer has already paid money in advance.