Single to high-count multilayer in every technological form.
Single Sided PCB
A single-sided PCB, by definition, has a single conductive copper layer which sits on top of a supporting dielectric, typically resin impregnated woven glass fabric (FR4).
Double Sided PCB
A double-sided PCB, by definition, has two conductive copper layers separated by an insulating dielectric, typically woven glass fabric (FR4). A double-sided PCB is different from a single sided PCB in that it will have Plated Through Holes (PTH) to take connections from top side to bottom side and vice versa.
A multilayer PCB will, by definition, have 3 or more conductive copper layers, each separated from the others using either fully cured core material, or partially cured prepreg material. The application of heat and pressure in the bonding press fully cures the prepreg material to create the finished multilayer. PTHs take connections from layer to layer in the same manner as a double-sided PCB.