Q: What is a merchant account?
A: Merchant account is similar to a bank account but with a purpose of holding funds received from credit and debit card sales taken by the merchant (that is you). Whenever a person with a credit card pays for something with their card, either offline or online, that money goes into a merchant account. From there the funds are wired out to a normal business bank account, either daily or on a weekly basis. So if you wish to accept card payments, you will need a merchant account; this can either be opened directly with the acquiring bank (direct merchant account) or you can use a 3rd party aggregated account (e.g. PayPal or aggregated merchant account).
Q: What is an acquiring bank?
A: An acquiring bank, also referred to as the acquirer, or processing bank, is an institution that opens and maintains a merchant account. It acts as a middleman between the credit card processor and the card associations (Visa, MasterCard, etc). Because the credit card associations are not dealing directly with merchants, a merchant account is needed at a registered acquirer. This account acts as a repository for your money. The acquirer also takes the risk that you as a merchant will remain solvent, which is why it gets down to the nitty gritty when reviewing your application to determine the risks and probability of refunds, chargebacks and positive account balance.
Q: What is a payment processor?
A: Because very often the merchant account and payment gateway are provided together, this term tends to be used incorrectly as merchants are misusing it when referring to a merchant account provider. In short, the payment processor is a company that processes credit card payments (Visa/MasterCard). Processor is actually more suited as an alternative term for a payment gateway, because it's role is to provide a gateway which handles the transaction requests and is integrated with different acquiring banks, various other payment gateways and service providers, distributing data between this network and the merchants.
Q: What is a payment gateway?
A: When you accept payment from a client, credit card details need to be sent through the network to the acquiring bank, Visa/MasterCard and issuing bank to check if the cardholder has enough money to pay for the transaction. In a traditional brick and mortar retail shop this happens through a POS terminal (Point Of Sale), a small machine that collects card data and sends it to the acquiring bank and Visa/MasterCard. However in an e-commerce (online based) sale, this process happens through a payment gateway.
In short, gateway is an online application hosted by the acquiring bank or a payment provider company. This application receives transaction requests from a merchant (for example via shopping cart) and is connected to Visa/MasterCard and at the end of the line to the cardholder's card issuing bank to check for sufficient funds. Commonly and ideally, merchant account and payment gateway are arranged and provided by a single company as a full stack solution.
Q: Difference between direct and aggregated accounts
A: When you open a merchant account directly with the acquiring bank or a merchant service provider, you'll be asked for detailed documentation and put through extensive risk assessment underwriting process. Once approved, you get your own MID (Merchant Identification Number) issued and a direct agreement with the acquirer, with your own custom billing descriptor (text that appears on your client's credit card statement). Direct MIDs are harder to get approved for and come with more friendly terms, such as faster payouts and lower rates.
For merchants who cannot obtain a direct merchant account, either due to higher risk category, being a startup or because of geographical limitations, can use a 3rd party merchant account instead (Aggregator, Master MID). This is a merchant account, typically owned by the payment provider company, sharing transactions from dozens of different merchants at the same time. Placement on such account is easier and much faster since there is no extensive approval process by the bank. Aggregated accounts are more expensive and may be riskier if you are dealing with the sketchy providers that are managing their MIDs recklessly by processing for very high risk merchants.