If a card must be tapped or flipped, it must be turned approximately 90 degrees (tapped) or 180 degrees (flipped), whichever is appropriate.
Flipped cards were introduced, and currently only in, Kamigawa Block. They consist of a card with two rules boxes, and have a condition to flip them from their original characteristics to alternate characteristics.
Whether a card is tapped or flipped is a status of the game, and needs to be clearly communicated with the opponent, and visual standards are very helpful in this regard.