A card type (with a lowercase "t") refers to the three main types of cards: Monster, Spell, and Trap.
Several more card types, including Virus, Trap-Spell/Spell-Trap, Illusion, and Equip, have been featured in the manga; a Spell variant, Speed Spell, has been featured in the anime and in some video games; and Ritual has been featured in some video games.
Cards that check card types, such as "Ordeal of a Traveler", usually specify "Monster, Spell or Trap" in parentheses to avoid confusion with monster Types.