Gaff cards are playing cards that have been doctored ion one way or another so you can do magic tricks that would be very difficult or impossible to do with an ordinary deck. I got The Blue Gaff Deck a few months ago and I love it. There are some amazing tricks you can do with it, and because they have the familiar Bicycle backs, no one will know that you are using gaff cards (as long as you don't flub).
You can do 40 different tricks with the deck
(and it comes with a DVD so you can learn them all) (a reader alerted me that they did not get a DVD. I got one in my deck). My favorite is the B-Wave, which is worth the price of the deck: