Accounts payable is usually described as the money a business owes its suppliers, which is the balance rather than the function. As a function it is the verification that sits between a supplier's claim and a payment: establishing that the claim is genuine, that it matches what was agreed and what arrived, and that somebody with authority accepts it.
As a balance
Money owed to suppliers and not yet paid, sitting on the balance sheet as a liability. That is the accounting sense, and how any particular invoice should be classified or accrued is a question for your accountant rather than for the payables team, whose contribution is accurate underlying detail and reliable dates.
As a function
Receiving invoices, validating them, matching them against orders and goods receipts, routing them for approval, coding them, paying them and maintaining supplier records. Seven tasks, of which the match is the central control and supplier record maintenance carries the highest fraud risk despite usually receiving the least attention.
What it is not
It is not purchasing, which decides what to buy and issues orders. It is not receivables, which is the mirror image chasing money in. And it is not general ledger accounting, which decides how a transaction is classified. The boundaries matter because most payables friction sits exactly where the function hands off to another one.
Why it is harder than it sounds
Because the two documents that make verification possible, the purchase order and the goods receipt, are produced by other people who are measured on other things. Payables inherits their quality and controls neither, which is why an efficient team can still be busy and why reporting that dependency matters more than absorbing it.
Questions people ask about accounts payable is
Is accounts payable an asset or a liability?
A liability: money owed and not yet paid. The specific treatment of any item is a question for your accountant.
How does it differ from accounts receivable?
Receivables chases money in from customers; payables verifies and pays money out to suppliers. Structurally similar, opposite incentives.
What should the function be measured on?
Queue state, the age of the oldest item, and the share of invoices that match cleanly, with the last understood as partly a measure of other departments.