These are the two steps in getting goods in from a supplier.
Purchase order
A purchase order is your order to the supplier: which items, quantities and agreed prices. It documents what you have ordered but does not by itself move inventory.
Goods receipt
A goods receipt registers that the goods have arrived. Here they are added to inventory so the stock level is correct. You can receive a whole purchase order or only part of it, if the delivery arrives in several batches.
What comes next
After the receipt usually comes a purchase invoice — the supplier's bill — which you can create directly from the receipt.
Read more in Purchase orders, Goods receipts and Purchase order / goods receipt (quickguides).