It is the physical embodiment of "no u". Comes in four colors, red, blue, green and yellow. There have been different styles with different itterations of the game. It is popular to use after someone calls you a fag or retarded, hold it up and say "No, you".
Often referred to as "No U" uno reverse card is a statement you make after another says they will do something to you. This causes them to do it to themselves.