Dice are what you roll when you're playing Yahtzee or Monopoly; they are two cubes marked with dots on each side that represent the numbers one through six.
The noun dice is the plural form of the singular die. Although many people use the word dice when they're talking about a single die, it's actually only correct to call two or more of the dotted cubes dice. You can also use the word as a verb to mean "chop into tiny pieces or cubes." You might, for example, read a recipe instruction that says: "Dice three tomatoes."