Some of these definitions are a bit loose, but should at least help clarify the use of these over-used words in the rest of the page.
Payment Processor Provider is a commercial entity that will sell you one or more payment processor services.
Payment Processor is a real-life service or process that enables a constituent to give money to the organization that owns the CiviCRM instance. Many payment processor service providers enable more than one payment processor. Not all payment processors need to be attached to a provider (in theory).
Payment Class is a php object class that represents a payment using a processor, and provides methods and properties that integrate the processor's functions into CiviCRM.
Payment Type is a CiviCRM-specific classification of payment types - an integer representing the 'type' of payment this payment processor class supports. Almost all existing payment processors are of type 1, for credit cards, but there is some support in code for 2 ('direct debit'). The primary use of this type is for generating a base set of fields in the user form.
Billing Mode is a CiviCRM-specific classification of different types of payment processing: 'form', 'button', or 'notify'. These determine the user workflow required and how the CiviCRM code works.
Online payment processing is a relatively recent innovation and is still evolving. The earliest widely used, and still most widely used payment processor provider is Paypal, and it has some pretty unique features both from a user and from a developer perspective. Most other payment processors created since then provided a simpler set of features, only work with credit cards, and mostly work in a similar way from a developer perspective - simply posting a set of named fields/values to an https url.
More recently, payment processors have started evolving in new ways to support mobile, swipe payments, and many kinds of non-credit card payments, and also have developed new ways to deal with security challenges.
The biggest issue for payment processor interface is the balance between customization and security. For example - the most secure kinds of interfaces are usually those hosted on the payment processor servers, but those are also the least customizable, and risk losing potential donors, for example. Hosting the payment page on your own site makes for a more customizable user experience, but creates new security demands on your whole server and code infrastructure. There are some new methods now evolving that support a kind of hybrid (e.g. Stripe, and iATS Direct Post method). There are a complex set of requirements called PCI for any server dealing with credit card payments.
Because the PCI requirements escalate dramatically when storing credit card numbers on your server, another key challenge of payment processors is to provide recurring payment functionality. There are two strategies here: control the recurring payments with the payment processing service provider, or use a 'token' based system that allows CiviCRM to control the amount and schedule of the payments, but use only a token to identify the payer.
A challenge for any tool like CiviCRM is to provide supporting code and infrastructure for these payment processors that are all quite diverse in their ways of working, needs, and language. Two useful examples that are relatively successful in this challenge are Drupal's commerce module, and the omnipay project.
CiviCRM's integration of a payment processor is based on subclassing the abstract class CRM_Core_Payment. That class is defined by the file CRM/Core/Payment.php, and each supported payment processor has a file in the directory CRM/Core/Payment that subclasses this abstract class. Any extensions that provide a payment processor have to provide a corresponding subclass file in their own CRM/Core/Payment directory.
The key method defined by the payment-processor-specific payment class depends on the billing mode. This is roughly documented on these two pages: Example of creating a payment processor extension and Create a Payment-Processor Extension.
The payment object is used in several places in the CivICRM code base. It's intention is to encapsulate all the details of processing a payment, so it can be used for a simple donation, a recurring series of donations, a payment for a membership, a payment for an event, and some combination of those (e.g. a recurring series of payments for an auto-renewed membership).
The way that payment objects get created is a bit convoluted. The payment objects are not created with the typical "new <classname>" command, but use a mangled version of the singleton pattern to create new payment objects, with no more than one per mix of mode, payment processor, and whether it's attached to a form. Specifically, they get created with this kind of thing:
Inside the singleton function,
So that means we are creating unique payment objects per distinct value of: mode + payment processor id + <is it associated with a payment form>, where 'mode' in this case just means test or not (not the "billing mode"!).
The paymentProcessor object is just the simple BAO object of fields from the corresponding database table, and the singleton function then finds the classname of the processor, and runs the singleton function of that class to create the actual payment object that subclasses the abstract CRM_Core_Payment class.
At the end of this process, you have a payment object that includes payment processor specific methods and a reasonably simple payment processor object attached to it.
Payment Processor Objects, and in the Database
Most object instantiations in CiviCRM correspond to a row in a similarly named table. Payment processors are also like this, but just a little more complicated.
The key tables are:
civicrm_payment_processor_type This table provides some db-configurable values for a "payment processor" as defined above. For example - the labels for the account name and passwd and the urls used with that service. Is also contains fields with default values for the billing mode, payment type and whether it supports recurring billing. Most importantly, it contains the class name of the payment processor that corresponds to the file defining the class.
civicrm_payment_processor This table provides the organization-specific implementation details of the payment processor - most notably the account name and passwd. It has a foreign key into the payment processor type table, and fields that allow for service-specific overrides of the billing mode, payment type and whether it supports recurring billing. This is the table that provides the row that helps instantiate the payment processor object.
Payment Objects in Code
As hinted at above, how the payment object is used in the code base is the most challenging part, particularly how it deals with the variety of ways that payment processors work.
In theory, all the details of how a payment processor works should be encapsulated as a payment object method or property. Unfortunately, a lot of core code has if/then clauses that make quite a lot of assumptions about the payment processor.
For an extreme example, in some cases there are bits of code in templates that check if a payment process or is paypal.
Some other examples are:
Here's a partial list of related discussions (please contribute here ..):
and examples of challenges with the payment processing code