What is a Card Scheme?*

Card Scheme is the company that provides the branding for credit card and debit card; as also the rules and infrastructure for processing of payment card transactions.

Also known as Card Network, the leading Card Schemes in the world are Visa, MasterCard, American Express, China Union Pay, RuPay, etc.

Barring AmEx, the other card schemes don’t issue their own cards or acquire their own merchants. The activity of issuing cards to consumers to use for shopping and acquiring merchants to accept those cards is done by banks (and some category of nonbanks). At one point in time, leading card schemes were jointly owned by banks but Visa and MasterCard are now independent, publicly listed companies.

You can get a high level understanding of the relationship between card scheme on the one side and issuers and acquirers on the other from the credit card value chain.

https://gtm360.com/blog/2017/07/14/how-to-fight-card-payment-surcharge-and-take-cashlessindia-to-next-level/#payment-card-value-chain
As this illustration makes clear, credit card is a 4-corner business. Ditto most digital payment business including Venmo (USA), FPS (UK), UPI (India). In this respect, a payment business is very unlike most traditional businesses, which are 2-corner businesses.