The Card Security Code ("CSC" for short) is a three- or four-digit number printed on the credit card for security purposes.
On most cards, the CSC is three digits long and is printed on the back of the card (usually in the signature field). Other numbers may appear to the left of the CSC.
American Express cards call the number "Card Identification Number" ("CID" for short). This is a four-digit non-embossed number printed on the front of the card, located above and usually to the right of the raised (embossed) account number.
In the pictures below, the circles indicate the CSC on the back of a card, or the CID on the front of an American Express card:
