A junk card is a card with no value or use in combat. It may have been made this way, as a joke, or it may have been modified or vandalized to become a junk card.
Junk cards, of course, cannot be used in a duel. However, they may be amusing to make or collect.
Cards that were made to be funny but still have usable attacks and effects (such as Allergy Ree) are NOT junk cards. A card is only a junk card if it has been made (or modified) to be completely useless.
Junk Card Slideshow[]
This slideshow shows examples of junk cards, most of which were produced by Ben and Jack.