Monster Card or monster
Monster Cards (Japanese: モンスターカード Monsutākādo) represent monsters that players battle against each other or directly against either player during the Battle Phase. Monsters are the main focus of Duel Monsters. Monster Cards are differentiated by their names, Type; Attribute; ATK; DEF; Level (or Rank or Link Rating). Monster Cards can be subdivided into Normal Monsters, Effect Monsters, Ritual Monsters, Fusion Monsters, Synchro Monsters, Xyz Monsters, Pendulum Monsters, Link Monsters and Token Monsters, each with a distinctive colored card frame to differentiate them.
In extent to the aforementioned classifications, Monster can also be classified as Tuner monsters or Special Summon Monsters, but this classification isn't a separate division from those above. Also, Effect Monsters, exclusively, can be classified further by Ability. This include Spirit Monsters, Toon Monsters, Union Monsters, Gemini Monsters and Flip Monsters.
Normal Monsters lack effects and are not typically paired with any other Monster Card types (with the exception of Gemini monsters, a class of Effect Monsters which are summoned as Normal Monsters, and some Pendulum Monsters, which can also be Normal Monsters). Effect Monsters cannot also be Normal Monsters (with the exception of the aforementioned Gemini monsters), but they can also be Ritual, Fusion, Synchro, Xyz Monsters, Pendulum, and Link Monsters. Some Ritual, Fusion, Synchro, Xyz, and Link Monsters lack effects and are thus not also Effect Monsters, but they are not considered Normal Monsters. Such monsters are known as Non-Effect Monsters.
During play, monster cards can be played in either face-up Attack Position or face-down Defense Position. The importance of monster cards lies in their effects (which usually activate during their controller's Main Phase) and their ATK and DEF values during the Battle Phase (or more specifically, the Damage Step).