# Order and Transaction Concepts
source: https://developer.mastercard.com/mastercard-gateway/documentation/getting-started/order-trans-concepts/index.md

The concepts of order and transaction help you to understand the general process of managing payments for online services:

* An **order** is one instance of a sale between you and your payer. It represents the classical paper 'order' concept, where you sell a bill of goods to your customers. It contains all the classical order data, which includes:
  * The line items for the goods that were bought.
  * The payer billing details
  * The used payment method
  * Where the goods are to be shipped
  * The amount paid
* A **transaction** is a specific action made on an order. It may be authenticating the payer or card or reserving or moving money between yourself and the payer. Warning: An operation within an API is the action of sending a request and receiving a response. In the context of the Mastercard Gateway APIs, each transaction operation either creates a transaction on an order, retrieves existing order or transaction information or a gift card or reward balance inquiry.

For more details about orders and transactions, see [Orders](https://developer.mastercard.com/mastercard-gateway/documentation/getting-started/order-trans-concepts/order/index.md) and [Transactions](https://developer.mastercard.com/mastercard-gateway/documentation/getting-started/order-trans-concepts/transaction/index.md).

For a quick introduction to identifiers used to track orders and transactions in various systems, see [Identifiers](https://developer.mastercard.com/mastercard-gateway/documentation/getting-started/order-trans-concepts/identifiers/index.md).

If you have no experience with payment processes and want to understand the basic actors and flows used when working with the Mastercard Gateway, see [Payment Actors and Flow](https://developer.mastercard.com/mastercard-gateway/documentation/getting-started/order-trans-concepts/pay-actor-flow/index.md).

## Series of payments {#series-of-payments}

A cardholder, generally for a one-off payment, or a merchant initiates the payment. A series of payments consists of merchant-initiated payments, where:

* The first payment is a cardholder-initiated payment. The cardholder gives the merchant permission to store their payment details and to make subsequent payments. Payment details are stored in compliance with Credential on File (COF) requirements.
* Subsequent payments are merchant-initiated payments. These payments use the stored payment details and the cardholder agreement to determine when and what kind of payments you can make. For more information, see [Cardholder-initiated and Merchant-initiated Payments](https://developer.mastercard.com/mastercard-gateway/documentation/gateway-features/cit-mit/index.md).