Have you ever wondered about zero percent financing or deferred-interest plans offered by some retailers?

If you've made a big purchase in the past few years, chances are you've been offered a deferred-interest plan along the lines of "no interest if paid in full in 12 months." The programs are common in stores that sell high-dollar merchandise such as furniture, electronics and other pricey goods. They're even popping up in doctors' and dentists' offices.

What you may not realize: That offer is probably a credit card, says Chi Chi Wu, staff attorney with the National Consumer Law Center.

Play by the rules, and you get a no-interest loan. However, if you go too late on a payment, misinterpret the payoff date or lose your ability to pay, you could end up with a hefty interest rate plus retroactive interest added to the bill.

"The key thing is that a deferred-interest program is a gamble against time," says Nick Bourke, director of the Pew Charitable Trusts' Safe Credit Cards Project. "The person who uses the deferred-interest offer is betting that they'll be able to pay off the entire balance before the end date."

It pays to ask a few questions and understand the finer points. Here are five things you need to know about deferred-interest credit cards.