Jane, sorry about that. I posted this a while back:

I like the CitiBank credit card virtual account numbers, which (1) are free and easy; (2) give 100% protection against future abuse.; and (3) are a free option/bonus feature that comes with the regular plastic CitiBank credit cards.

Info here:
http://www.citibank.com/us/cards/cardserv/advice/van.htm

it is easiest to give an example. Let's say your regular plastic credit card number (visa, mastercard, whatever, issued by CitiBank) is #12345...... You want to buy cigars for $58.89 on the internet but are afraid to give an unknown outfit (or anyone) your regular credit card number (#12345.....). You log into your #12345 account at CitiBank web site, and click to get a one-time virtual account number, which is generated at random just for you just for this occasion. There are lots of options, but generally you would just type in an amount.

So in this case, you could end up with $60.00 (or whatever), an account number (say, #2468....), a three-digit CVS/pin number (say, 759), and an expiration date (which will be the next month, say 01/06). You then copy/paste this and go to the cigar site and give them that info. End result:
(1) The cigar place can use that credit card number (#2468) just once, and only up to $60.
(2) The cigar place never can know that you have an account #12345.
(3) the $58.89 charge will automatically be billed by CitiBank to your regular account #12345.....
(4) The credit card #12345 can be used anywhere like any credit card, but the virtual numbers can only be used on the internet.

Robert