When your card is "skimmed," somebody steals information by running your card through an electronic reader. They gather card numbers and other details, which they can use or sell later. You'll have your card in your pocket, so you won't notice anything until they've had a chance to spend a few bucks.
Where exactly does skimming take place? BankInfoSecurity.com lists four of the most likely card skimming culprits.
You don't even have to let a card out of your sight to get skimmed - it can happen at an ATM or gas pump. It's a good idea to be careful with your cards, but they're no good if you can't and don't use them. If skimming is not a recurring problem for you, it might make sense to just use common sense and review bank transactions regularly.
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