Sluff Off! follows the standard trick-taking formula: The cards are numbered 1-15 in five different suits, and blue is always trump. If someone leads a color, you must follow suit if you can; otherwise, you can throw off a card or trump the trick.
It's an exact-bidding game, but you predict your tricks by taking colored chips. For each trick that you win, you toss a chip back. If you take a trick in a color for which you didn't have a chip, then you must take a black chip (worth -3 points). If you have any colored chips left at the end of the round, they're worth -2 points. If you have any white chips left at the end of the round (wildcards that you get when someone takes a colored chip away from you during bidding), they're worth -4 points.
There's also the Sluffer, whose role is to feed unwanted tricks to other players. Instead of bidding tricks the Sluffer takes 4 black chips each worth -1 for him at the end of the hand. When players take tricks they didn't predict, they take black chips from the Sluffer. If the Sluffer feeds enough of the black chips to other players, it can zero out the Sluffer's score for that round.