Union Monsters are Effect Monsters with the "Union" ability. Each Union Monster has an Effect that allows it to equip itself to a Monster as an Equip Spell Card or unequip itself to Special Summon itself to a Main Monster Zone.
In order to equip a Union Monster to a Monster the Union Monster must be on the field.
Not all Monsters that can treat themselves as Equip Cards are Union Monsters. For example, "Dark Necrofear" has the Effect to turn it into an Equip Card, but it's not a Union Monster.
Union Monsters can only be equipped or unequipped once per turn.
Union Monsters can also be Tuner Monsters.