While credit cards can be extremely useful, they also have their risks. If you don't keep control of your spending, you can end up carrying a balance and getting charged interest, something that happened to two-thirds of cardholders over the last six months.And here's where it gets even worse-- according to a recent CreditCards.com survey, only 39% of those cardholders were sure about the annual percentage rates (APRs) on all their credit cards.