The core is the puzzle's centre. It usually holds the outer pieces of the puzzle together and is normally not visible unless the puzzle's outer pieces are removed.
More recently, different cores have been produced, Gancube made their octagon core which has been involved in their designs ever since. The core of a cube has to be well manufactured to prevent the screws from breaking the core when it is tightened, the moulding has to be very accurate.
All 3x3x3 cores made by MoYu are blue.