When you select Charge in the Process Payment Action, you are given two choices on how to collect the payment details:

UI Widget - Collect payment details from the user who enters it in manually. If the user has multiple cards stored, they can select a card without re-entering the details manually.
Stored Payment Details - Use the stored payment details from the payment gateway which was entered in earlier from a previous payment.
In both cases, you will be able to capture the following responses from the Payment gateway:
Gateway Response - The result of the request. Responses include, but not limited to, Success, Failed, and Pending.
Successful (True/False) - Whether or not the request was successful.
Reference Number - Payment transactions reference number provided by the gateway. Only provided after a successful transaction.
Charge using UI Widget
When this type is chosen, the user will have a prompt to enter in their payment details.

Payment is for specific Entity - For anonymous payments, for example, an anonymous donation, you can uncheck this option. When checked, you will also need to specify:
Customer RecordID
Additional options
Amount: The amount for the payment. Note, if your payment gateway does not support the specified currency the payment may fail.
When Payment is for specific Entity is checked, a number of other fields appear.
Customer RecordID: This requires the Record ID of the Customer who will make the payment.
Configure payment methods:
All payment records
ACH (or e-check)
Credit card
Save payment details - options for saving the customer's payment details:
Do not save details - Never save the user's payment details.
Allow details to be saved - Give them an option to save the payment details for the entity.
Always save details - Always save the details.
Charge using Stored Payment Details
When this type is chosen, the payment will be processed using the stored payment detail of the entity.

Entity RecordID Stored: The Record ID of the Entity (customer/vendor) who has stored payment details.
Payment Method ID (Optional): If the entity has more than one card saved, you can specify which one to use. If left blank, the first card will be used.
Amount: The amount for the payment. Note, if your payment gateway does not support the specified currency the payment may fail.