An invoice is a supplier's formal request for payment, stating what was supplied, what is owed and by when. Everything that happens afterwards is the buyer establishing whether the request is correct, authorising it and paying it. Understanding the sequence from the buyer's side explains most of the questions suppliers ask and most of the delays they experience.
What an invoice contains and why
The supplier's details, the buyer's details, a unique invoice number, the date, a description of what was supplied with quantities and prices, any tax, the total, and payment terms with a due date. Many also carry a purchase order reference, which is what lets the buyer match it. Missing that reference is the single most common cause of a slow payment.
What the buyer does with it
Checks it is genuine and addressed to them, checks the invoice number has not been seen before, compares it against what was ordered and what arrived, routes it to whoever has authority for that spend, records the approval, codes it and schedules it for payment on terms. Each step is quick; the routing step is where the elapsed time goes.
Why payment takes longer than the work suggests
Almost never because anyone is holding money deliberately. Usually because the invoice could not be matched, because it went to an individual rather than a central address, or because the approver was unavailable. All three are addressable, and suppliers can help with the first two simply by quoting the order number and billing the right address.
Questions people ask about how do invoices work
When does the payment clock start?
Usually from receipt of a correct invoice by the buyer, which is why the date it arrived matters and why disputed invoices restart conversations about terms.
What makes an invoice invalid?
Practically, anything that stops the buyer identifying what it is for: no order reference, wrong entity, or amounts that do not correspond to anything supplied. The specific legal requirements depend on your jurisdiction and tax position.
Why do buyers ask for a purchase order number?
Because it is what connects the invoice to what was ordered and what arrived. Without it the invoice must be investigated by a person instead of matched.