A purchase order and an invoice cover the same transaction and are written by opposite parties at opposite ends of it. The order is the buyer's statement of what was agreed, written in advance. The invoice is the supplier's claim for payment, written afterwards. Almost everything a payables team does follows from comparing the two.
Written by the buyer, in advance
The order lists items, quantities, unit prices, a delivery address and date, your terms, and a unique number. It records the agreement and becomes a commitment once the supplier acts on it, which is why approval belongs before issue. It is also your only contemporaneous evidence of what was agreed, so its accuracy matters later.
Written by the supplier, afterwards
The invoice states what was supplied, what is owed, by when, with their own reference and ideally your order number. It is a claim rather than a fact. Business buyers verify claims before settling them, which is the difference between payables and consumer payment and the reason the process has more steps than people expect.
How they are compared
Supplier, unit price, quantity and total, at line level, within a tolerance that absorbs the small legitimate differences: rounding, freight added at billing, part deliveries. Anything outside tolerance becomes an exception carrying its reason, routed to whoever can resolve it rather than into a shared queue nobody owns.
And the third document
An order and an invoice agreeing proves the supplier billed what you asked for and says nothing about delivery. The goods receipt closes that gap, which is why the standard control for physical goods compares three documents. Without it, you are trusting the supplier on exactly the point most worth checking.
Questions people ask about difference between purchase order and invoice
Which comes first?
The order, always, where one exists. An order raised after an invoice arrives is documentation rather than a control.
Can they legitimately differ?
Yes: freight, tax, part deliveries and agreed price changes. Tolerance and dated revisions handle those without creating exceptions.
What if there is no order?
The invoice is approved on its own merits by whoever authorised the spend, which is the most expensive path through payables.
Should the order number appear on the invoice?
Yes, and it is the single most useful thing a supplier can do to be paid promptly, because it is what lets the invoice be checked automatically rather than investigated by a person.