Gaffed cards are trick-cards that can be for example double sided or have non-original faces. These types of "fake" cards can be used for a great variety of card magic-tricks. With sleight of hand, you can make the spectator believe that you have an original deck of cards. These kinds of illusions will be more powerful, giving a better reaction from the spectator(s).
No way Josh! How did you do that trick?
I'm not telling...
Ok, i used gaffed cards i bought on the internet.
You cheeky little bastard! You fooled me so bad!!! Haha.