What is a PO vs. invoice?

A PO comes from the buyer before delivery ('here's what we're ordering'); an invoice comes from the seller after delivery ('here's what you owe us'). They describe the same transaction from opposite ends: the PO is a request and authorization to buy, the invoice is a request to be paid.

They work as a pair: the invoice should match the PO's items, quantities and prices — and checking that match (plus what was actually received) is how businesses catch overbilling, double-billing and price creep before paying. No PO means the invoice gets compared against someone's memory.

One more difference that matters: the PO becomes binding when the supplier accepts it; the invoice is not a contract at all, just a bill referencing one.