Both "The SD Card" and "SD Wallet" are prepaid systems which allow you to pay for value-added / premium services available at streetdirectory.com. You need to be a member to use SD Wallet. SD Card are for non members.
When you use the "SD Wallet", you get more dollar value credited into your "SD Wallet" account e.g for every S$20.00 purchased, you get 23 SD Dollars credited to your "SD Wallet".