Yu-Gi-Oh! is an official card game, or OCG made by Konami. Players "duel" by putting a combination of monster playing cards against one another. Each monster card and type has a history that is introduced and expanded upon by the anime series. Fusion Monsters are a type of Yu-Gi-Oh! playing card that represent the combined effect of two or more monsters on one card.
Fusion Monster Card Details
Fusion Monster cards are always colored purple and can only be placed in your Extra Deck along with Synchro Monsters and Xyz Monsters. An advantage of this placement is that the contents of your Extra Deck are not public knowledge as is the case with your Main Deck of playing cards. Fusion Monster cards were originally created by the Millenium Scale, a magical device in the Yu-Gi-Oh! anime that has the ability to trap souls.
Fusion Monsters can be summoned, or used during game play by invoking a Fusion Summon, which requires the use of a special combination of cards including Polymerization and the Fusion Material Monsters, which are listed on the Fusion Monster card you want to use. The Fusion Material Monster cards for each Fusion Monster vary. For example, to Fusion Summon the Flame Swordsman, you must use a Polymerization spell along with the monsters Flame Manipulator and Masaki the Legendary Swordsman. When this is done, both the Flame Manipulator and Masaki the Legendary Swordsman must be transferred to the Graveyard (where they cannot be used) just before you use the Flame Swordsman.
Additional Summoning Guidelines
You do not always have to use the Polymerization method to invoke a Fusion Summon. The Yu-Gi-Oh! OCG and anime outline a list of acceptable spell cards that you can use for this purpose. Examples include the spells, Dark Calling, Dark Fusion and Future Fusion (see Resources for the extensive list). While you can use a Special Summon to invoke a Fusion Monster, this method does not qualify as a Fusion Summon, which means the monster loses some of its ability.
Exceptions (Summoning Conditions)
Fusion Monsters that end up in the Graveyard, the place for expired cards, cannot be Special Summoned back to life with normal spell cards such as Monster Reborn. Fusion Monsters can be brought back only with Polymerization-based Fusion Summons unless you have a card that states, "this Special Summon is treated as a Fusion Summon." Finally, some Fusion Monsters like the Elemental HERO have conditions stated on the card that prevent them from being Special Summoned.