MasterCard payments at online casinos
Initially the company's card was named Interbank/Master Charge and was created by a few Californian banks in 1966 as a competitor to BankAmericard, which is known these days as Visa. In 1979 Interbank became MasterCard.
Once your credit provider, usually your bank, approves you and issues you with a MasterCard you can use it to make online purchases such as depositing to an online casino account. The casinos will need to have the MasterCard logo showing that they accept MasterCard. Your purchase is an agreement with MasterCard to pay whatever amount you have charged on your card. You authorise the purchase by entering your PIN number or in case of online casinos just your credit card information, as casino will use the CNP (Cardholder Not Present) transactions. Through an electronic verification system the online casino will check if you have available credit and process the payment. At the end of the month MasterCard will bill you and you will need to pay a minimum amount owned to them by a certain date.
MasterCard Worldwide also issues a debit card branded as Debit MasterCard (formerly known as MasterMoney). This is connected to your bank account and there is no credit facility, so payments require a sufficient balance in your account at the time of the payment to cover the purchase. Due to the potential for the abuse of credit by problem gamblers, players in some jurisdictions (notably the United Kingdom as of April 2020) cannot legally use a credit card to deposit at an online casino. However a MasterCard branded debit card attached to a bank account where adequate funds are held at the time of the transaction will usually be accepted without issue.
Further information about MasterCard is available here: MasterCard website