Unlike a Double-Sided PCB which only has two conductive layers of material, all multilayer PCBs must have at least three layers of conductive material which are buried in the center of the material.
Benefits of Multilayer PCBs
Smaller size (considerable savings on space)
Easier incorporation controlled impedance features.
EMI shielding through careful placement of power and ground layers.
Reduces the need for interconnection wiring harnesses (reduces overall weight)