Fraud is any transaction that is not made by you. There are several forms of fraud.
Lost or stolen card.
Your card is lost or stolen and the card is subsequently used without your permission.
You did not receive a new or replacement card that was sent to you by your Issuing bank. You may have no idea that the card was missing until you receive a statement listing transactions not made by you.
Your personal information is used to apply for a credit card. This type of fraud is often difficult to detect until the issuer receives a complaint from the consumer or the account goes into collections shortly after having been issued. If you're not a customer of the bank that issued the card, you may not know a card has been issued in your name until you apply for credit elsewhere and are denied because of a poor credit rating.
Even though the card is issued to you by your bank, transactions appear on your account statement not made by you. This could be an indication that a counterfeit card bearing your account number is in circulation and being used at the same time you're making legitimate purchases.
While you're in possession of your card, somebody else posing as the cardholder "takes over" the account by requesting a replacement card on the same account, usually to be mailed to a different address. You would not know this has been done until you receive an account statement showing transactions not made by you or you fail to receive your monthly account statement.
No card present
While you're in possession of your card, somebody makes transactions using the card number only, as in the case of mail orders, phone orders, or Internet transactions. You may not know this has been done until you receive an account statement showing mail/phone order or online transactions not made by you.
There are other scenarios that are classified as fraud, but don't fall into the described categories. An example would be the theft and subsequent use of balance transfer checks, which you may or may not have ordered from your bank.
What to do if you suspect fraud
Call the bank or financial institution that issued your card immediately. Your issuer may want to cancel your current card and issue you a new one. Check with your issuer to verify that your mailing address has not been changed.
If you still have your card but fraudulent purchases have been made, call your issuer to report the fraud and request a new card.
Also, contact the credit bureaus to let them know that fraud has occurred. A "Fraud Alert" message will be placed on your file. You should also request a copy of your credit report and review it carefully.