Armor Monsters (アーマーモンスター Āmā Monsutā) are a sub-class of Effect Monsters that were only seen in the Waking the Dragons arc of the second series Yu-Gi-Oh! anime, but were eventually expanded into a way to improve the interactivity of Dueling.
As their name implies, Armor Monsters take the form of pieces or full sets of armor that equip themselves to the Duelist who played them rather than actual monsters. This allows the Duelist to attack the opponent's monster themselves, either physically or by whatever weaponry their armor has. When used correctly, Duels with Armor Monsters can be highly interactive and exciting, but as the Duelist can physically attack opponents, the Duelists are in real danger of being struck down.
In-game, Armor Monsters share two effects: Only one Armor Monster can attack per turn, and if there are two or more Armor Monsters on a player's side of the field and one of them is declared as an attack target, that player can change the target to another Armor monster he/she controls. The reason for this is that for the former, because the Duelist themselves is attacking, they can only attack once, and for the latter, the Duelist can defend themselves with whichever armor piece they want (or, in the case of "", a redirection program is used).
In the Yu-Gi-Oh! GSTART era, however, a new breed of Armor Monsters were created that do not equip to the Duelist, but rather fill in the pictured armor with the Duelist's D-Vatar, virtually turning the Duelist into a monster to battle the opponent. The common effects of Armor Monsters are explained for this type of Armor Monster as such: Only 1 Armor Monster can attack because the Duelist can only use their main D-Vatar to fill in an armor, thus only having one D-Vatar that can attack, and the player can change the attack target to another Armor Monster by having their D-Vatar quick-change armors in the face of the attack.