When you shop using a credit card you are borrowing money and paying a bank or financial institution back at the end of the month or so. But as you pay them back there is an interest along with it so say you spent 200 dollars of there money but its 6% interest then you have to pay that and possibly more judging on who you pick. So its best to search around and find the cheapest one and also make sure you pay it back on time to boost credit as well.