The Forbidden Fruit grew on the Tree of Knowledge of Good and Evil.
After being tempted by Satan (in the form of a snake) Eve persuaded Adam to eat of the fruit.
After God discovered what they had done he cast them out of the garden and cursed mankind.
Traditionally, the Forbidden Fruit has been depicted in art and other visual media (movies, cartoons, etc.) as an apple, though the Bible does not identify it as such.