Track 2 data
The compact string of digits on a magnetic stripe's second track, holding everything a card reader needs — and everything a card forger needs.
The format comes from the American Bankers Association and is specified in ISO/IEC 7813. The track holds at most 40 characters, digits plus a few control symbols: a start sentinel, the card number, the separator =, expiry as YYMM, a three-digit service code and a field of issuer discretionary data where the CVV lives. An end sentinel and a checksum close it off. Almost every payment system reads track 2, because it is shorter and easier to parse than track 1 with its cardholder name.
Which is exactly why track 2 is what skimmers harvest and what carding forums sell as "dumps". PCI DSS forbids storing it after authorisation. Chip cards carry an equivalent, Track 2 Equivalent Data (tag 57), and that is why EMV fallback to the stripe remains a soft spot.