Once your merchant account is set up, you will need some way of accepting card payments from your customers.
That might be some physical card terminals/electronic point of sale hardware for 'Cardholder Present' face to face transactions, to expect chip and pin and contactless payments.
For e-commerce/website transactions you will require a payment gateway, which is sometimes referred to as a PSP (payment service provider).
A payment gateway is an online service that securely transmits card information, subject to necessary fraud checks and verification of available funds, and debits the customer’s account for the required amount.
For payments over the phone, these types are payments are known in the industry as MOTO (mail order/telephone)
Sometimes these are also referred to as 'Cardholder Not Present' or CNP for short.
This means that the cardholder/customer is not present to use Chip & Pin to authenticate the transaction as they would in a physical shop, often using their 4 digit pin code to prove the card belong to them.