In Defense Position (called Defense Mode in the anime), a Monster Card can be either face-up or face-down, in a Horizontal Position. In this position, the DEF value is used to represent this card, except where a card effect requires a specific examination of the ATK value.
In Defense Position, a Monster Card is always in a Horizontal Position. When a monster is "Set", it is played in face-down Defense Position. Monsters cannot be Normal Summoned in face-up Defense Position, except by a card effect that allows them to be summoned that way (such as "Light of Intervention"). During battle between a Defense Postion monster, the DEF value of that monster is used to calculate Battle Damage, rather than the ATK value.
Usually, Defense Position monsters cannot attack, but some monsters such as Elemental Hero Rampart Blaster and Total Defense Shogun have effects that allow them to attack while in Defense Position.
In the Yu-Gi-Oh! 5D's anime, Defense Position monsters become colored blue.