It's a bit long, but worth reading! 
Answer:
A. Customer's name
B. Customer's partial home address
C. Hotel room number
D. Check-in date and out dates
E. Customer's credit card number and expiration date!
When you return them into the front desk your personal information is there for any employee to access by simply scanning the card in the hotel scanner.
An employee can take a hand full of cards home and using a scanning device, access the information onto a laptop computer and go shopping at your expense.
Simply put, hotels do not erase the information on these cards until an employee re-issues the card to the next hotel guest.
At that time, the new guest's information is electronically "overwritten" on the card and the previous guest's information is erased in the overwriting process.
But until the card is rewritten for the next guest, it usually is kept in a drawer at the front desk with YOUR INFORMATION ON IT!
The bottom line is:
Keep the cards, take them home with you, or destroy them. NEVER leave them behind in the room or room wastebasket, and NEVER turn them into the front desk when you check out of a room.
They will not charge you for the card (it's illegal) and you'll be sure you are not leaving a lot of valuable personal information on it that could be easily lifted off with any simple scanning device card reader.
For the same reason, if you arrive at the airport and discover you still have the card key in your pocket, do not toss it in an airport trash basket.
Take it home and destroy it by cutting it up, especially through the electronic information strip!
