To say that a card is face-down is to say that the back part of the card is facing up, and the face of the card is facing down. This is the opposite position of the card being face-up. Cards are usually put face-down via the act of Setting.

Face-down Monster Cards can only be in horizontal position (Defense Position) unless with the effect of "Darkness Approaches", you can have it in vertical position (Attack Position).

When a Spell or Trap Card is face-down, it is always in vertical position.

When any card is face-down, unless specifically mentioned by a card effect, its controller is not permitted to disclose any details of that card to their opponent, even if that card was face-up on the field a short time ago. Shuffling face-down cards can disrupt gameplay and is not permitted, unless the effect of a card, like "Shifting Shadows" and "Wandering Mummy" allows it.

Face-down cards have no name, Type, Attribute, ATK, DEF, Level until they are flipped face up.

Face-down cards cannot carry counters. Flipping a card face-down removes counters.

If a card only specifies face-down cards without saying where, it is implied to only mean face-down on the field (e.g. "Granmarg the Rock Monarch").

Monster Effects cannot be activated while that card is face-down unless it specifies on that card (e.g. "Big Shield Gardna", "Mid Shield Gardna", "Sand Moth", etc.).