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.

 

Multilayer PCBs have many advantages compared to single or double-sided PCBs, like: Increased flexibility, Higher assembly density, Smaller size, EMI shielding through careful placement of power and ground layers, Easier incorporation controlled impedance features, etc.