Monster cards form the most common cards in the deck. They can be found by their yellow, orange, white, black, blue, or purple background.

Yellow: Standard
Orange: Effect (does something besides attack/defend)
Purple: Fusion (requires specific other monsters to play)
Blue: Ritual (requires a corresponding spell to play)
White: Synchro (requires a tuner and other monster with levels adding up to the Syncro's level to be summoned.)
Black: XYZ (requires multiple monsters of the same level to be played)