Xynchro Monsters (pronounced eks-INK-row), also called Xyz Synchro Monsters, are a special set of Synchro Monsters, which are like normal ones, but have a special style. Xynchro Monsters require one or more Xyz Monsters as a material, and thus they have a Rank as well as a Level. Due to this, though, they can be affected by cards which affect Levels AND Ranks, and since both count towards the star limit, it is impossible to have a Rank or Level 12.
They gain the Xyz Material Monsters of the Xyz Monsters used to Special Summon it, which can be used for their own powerful effects. Note that Xynchro Monsters are not Xyz Monsters, and are not affected by cards which specifically designate Xyz Monsters. Xynchro Monsters can also have Spell or Trap as their Attribute, which isn't possible with any other non-Spell Monster or non-Trap Monster. Xynchro Monsters always have the same beginning text: All Xyz Materials on all Xyz Monsters used to Synchro Summon this card are added to it. These monsters are even rarer than Vortex Synchro Monsters, and yet even more powerful.