To say that a card is face-down, is to say the back part of the card is facing up. When a Monster Card is face-down, it can only be horizontal (i.e., in Defense Position). You can only have a face-down monster in Attack Position by the effect of Darkness Approaches.
When any card is face-down, unless specifically mentioned by a card effect, its controller is not obligated to disclose any details of that card to his/her opponent, even if that card was face-up on the field a short time ago.
Monster Effects can not be activated Face-Down unless it says so on the card (ex.Big Shield Gardna).
Types of Monster Cards
Types of Spell Cards
Types of Trap Cards
Other Types of Card
ATK - DEF - Level - Attribute - Type
Playing Field Zones
Types of Summon
Types of Monster Effects
Types of Effect Monsters
Types of Spell Speed
Aqua - Beast - Beast-Warrior - Dinosaur