Your debit card has got the Visa (or Mastercard) logo on it, it's got the magnetic strip, and it even has the right number of digits... but it's not a credit card. Think twice before you use it!

http://redtape.msnbc.com/2007/09/paper-or-plasti.html

Turns out debit cards don't have the same liability restrictions as credit cards. Keep this in mind when you make purchases on the net.