If you intend to sell goods online, you need to decide how customers can pay you. The more alternatives you can offer, the more likely you are to make the sale. Purchasing cards are credit cards issued by corporations to trusted employees.
Digital cash requires both customers and merchants to have accounts with the same issuing bank.
Hasn't quite caught on yet, but may in the future. Micropayments similar to digital cash but in very small denominations. Has a bright future. Electronic checks are just like paper but still in the testing phase. VirtualPIN is a kind of firewall for credit card numbers.