Issuer, acquirer, processor: who is who?
Four institutions touch every card payment and almost nobody can name them in the right order.
A stadium does not own the teams or sell the shirts. It owns the ground, decides who may play on it, and employs the officials who settle arguments. Everyone who wants a match has to book the pitch.
The same five parties appear twice. Once to ask permission, once to move the money.
The cast, in the order they appear
The cardholder is you. The merchant is the shop. The issuer is the bank that gave you your card, holds your account, and decides whether to approve each payment. The acquirer is the merchant's bank, which receives the settled funds and deposits them into the business account.
That is the whole cast. Everything else in payments is a service sitting between them.
The fifth thing that is not a party
The card network is not one of the four. It is what sits between them: the standards everyone speaks, the routing that carries the messages, and the rulebook that governs disputes.
Which is why a card issued in one country works in a taxi in another. Both ends speak the same protocol, written and maintained by the network. Networks do not lend you money and do not hold your money. They own the road, write the traffic rules, and run the courts. That turns out to be a better business than either.
The companies that sit between the four
A processor operates the technical plumbing that carries messages between these parties, often invisibly, often bundled inside another company's product. A gateway captures payment details at the point of sale or checkout and starts the instruction on its way. An aggregator or payment service provider bundles gateway, processing, and access to acquiring into a single contract, which is why most small businesses experience all of this as one company.
When it breaks, the name tells you who can fix it
When a payment fails, the failure belongs to somebody specific. An issuer decline means the customer's bank said no, and no amount of shouting at your provider will change it. A gateway timeout is a technical failure with a different fix entirely. A settlement delay is an acquirer question.
An issuer decline and a gateway timeout produce the same customer-facing failure message and require completely different fixes. Knowing which of the four you are dealing with converts a frustrating mystery into a solvable problem.
Where you meet it
Every card transaction. And in every contract you sign if you accept payments, where knowing which job the counterparty actually performs determines what they can and cannot fix for you.