A purchase order and an invoice are the same transaction seen from opposite ends. The purchase order is the buyer saying what they are ordering, sent before anything happens. The invoice is the supplier asking to be paid, sent after. Confusing them is common and it matters, because the whole point of running purchase orders is that the second document can be checked against the first. This page sets out the difference, the order they come in, and what each one commits the two sides to.
Direction and timing
The purchase order goes from buyer to supplier and comes first. The invoice goes from supplier to buyer and comes after the goods or services. Between them sits the delivery, and often a delivery note. That sequence is the reason a business that issues purchase orders can check an invoice at all: there is an earlier document, agreed by both sides, saying what was to be supplied and at what price. Without it, the invoice is the only record of the transaction and the supplier wrote it.
What each commits you to
An accepted purchase order generally forms a contract on the terms stated, which means the buyer is committed to take and pay for what was ordered on those terms. An invoice does not create the obligation; it asks for payment of one that already exists. This is a general description of how sales contracts work under the Uniform Commercial Code linked below rather than legal advice, and your own terms and state decide the specifics. Practically: the negotiating happens before the order, not when the invoice lands.
The po on the invoice is the join
When the supplier quotes your PO number on their invoice, the two documents become checkable against each other: does this number exist, is the amount within what was approved, was it received. That check is the entire operational benefit of a purchase order system, and it fails quietly whenever the number is missing. An invoice with no PO number is not a formatting problem, it is a purchase nobody approved, and it is worth treating as one.
When a PO is called a contract or an agreement
People search for po contract and po agreement meaning two different things. Sometimes they mean the question above, whether the order binds them, and the answer is that once accepted it generally does. Sometimes they mean a blanket arrangement, where one order covers repeated deliveries over a period against agreed prices. Those are different objects and worth keeping distinct in your own record: one is a single order, the other is a standing arrangement that individual releases are drawn against.
Questions people ask about po vs invoice
Which comes first, a PO or an invoice?
The purchase order, from buyer to supplier, before the goods. The invoice comes from the supplier afterwards, asking to be paid for what was delivered.
Is a purchase order the same as an invoice?
No. They are opposite ends of one transaction. The PO is what you ordered and agreed; the invoice is what the supplier says they delivered and want paying for.
Why does the PO number appear on the invoice?
So the invoice can be checked against the order: that the number exists, the amount is within what was approved and the goods were received. That check is the point of running purchase orders.
Is a PO a legally binding agreement?
Once a supplier accepts it, a purchase order generally forms a contract on the stated terms. That is a general description rather than legal advice; your terms and your state decide, and it is a question for your own adviser.