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If you want to manually enter your credit card transactions, then you don't. But if you want online instantaneous purchasing by your visitors, then you need an online payment gateway. The credit card payment gateway communicates with the banking institutions, securely communicating the purchase and credit card info to the bank. The bank sends approval (or denial back to the online payment gateway. Funds are then debited against the credit card and deposited into your merchant account. If you do it manually, you can manually enter your credit card transactions and process them like you would if you had a brick and mortar store.