Copy from the Yu gi oh wikia
Union Monsters can be recognized by the fact that, where on an ordinary Effect Monster it would read 'Effect', Union Monsters have 'Union'. Cards such as Dark Necrofear or Kiseitai act as Equip Spell Cards but do not have the text designating them Union Monsters.
Union monsters are a special type of Effect Monster with (Type)/Union in their card text. They can function like an Equip Spell Card, and thus become vulnerable to cards that destroy Spell Cards, such as Mystical Space Typhoon. They equip to the host Monster their text designates and generally grant it additional strength, such as an increase in ATK/DEF. All Union Monsters Destroy themselves to protect the host Monster if it would be destroyed by battle while equipped. The only exceptions are Heavy Mech Support Platform, Spirit of the Six Samurai, and Armored Cybern which protects the monster from destruction by both battle and Card Effects
Union Monsters can either Equip or Unequip to the host once per turn with their own Effect, given they are on the field. Additional cards, such as Formation Union and Combination Attack allow them to Equip and Unequip alternatively.
The Union Monsters introduced the XYZ Archetype (which would later become the VWXYZ Archetype), which had a new way of using their Fusion Monsters. This unorthodox method involves removing the Fusion Material Monsters from play to Special Summon the Fusion Monster from the Extra Deck (This is not considered a Fusion Summon). Polymerization cannot be used to Fusion Summon these monsters.
Here is a list of union cards:
(Cards that the union monster can equip to will be listed in parentheses after the name of each monster)
Custom Union cards Majesties' Throne + (any Majesty card)
Blade Enforcer - Jelly Man + (any LIGHT Blade Monster)
Blade Enforcer - Shark Dolphin + (any WATER Blade Monster)
Blade Enforcer - Jetcopter + (any WIND Blade Monster)
Blade Enforcer - Cannon Arm + (any FIRE Blade Monster)
Blade Enforcer - Earth Kid + (any EARTH Blade Monster)
Blade Enforcer - Weird Top + (any DARK Blade Monster)
Honey Collecting Wasp + (any insect monster)
Intellegent Weapon from Eberr + (any Eberr Monster)
Wayward Whelp Shepherd + (any Dragon monster)
Raptor - Deinonychus + (any Raptor Monster)
Arcana Heart - Catherine + (any Arcana Heart Monster)
Thunderous Hound + (any Lightning Deity monster)
Combination Attack Frontline Base Formation Union Roll Out!