A purchase order is a document, which the buyer sends to a supplier. A purchase order includes the list of products or goods (or services in some cases) that the buyer wants to buy from the seller, their quantity, supply price, date of creation and shipment date, shipping address, and other useful information for this order.
Invoice is a document, which the supplier sends to the buyer. It includes actual information about the list of products, their quantity, and price. Usually, the invoice is sent after both the buyer and supplier completely agrees and confirms the purchase order. Therefore, in some cases, the purchase order can be changed to actual quantity and price of products.
After the invoice is paid, the supplier sends the products to the buyer.