First, a double-board PCB and single sided PCB are essentially the same. The difference being is that a single sided board has copper on one side of the board. Technically speaking, a single-sided board will not have copper plated in the drilled holes either.
A multilayer PCB has more than two layers. That is, anything more than a double sided PCB.
Copper Clad FiberglassLet's start with the raw materials that make up PCBs. The foundation of the PCB is the rigid fiberglass laminate. When it comes to the manufacturing process, there are many types of PCB materials that may be used. Which material gets used is determined by the PCB designer in order to meet electrical, temperature, and other related properties. Fiberglass often referred to as FR4.