Plated Through Hole circuit boards are made by a complicated chemical process. Although the process is additive it is normal for the PCBs to start with a thin layer of laminated copper. This to date has proved the most reliable technique to ensure a good final peel strength. [Peel strength is the amount of force required to literally peel the copper from the base laminate.]    The basic procedure is to make the inside of the holes conductive either with thinly plated with copper, or carbon. We then print a negative photo resist of the desired track onto the panel, and plate up to the desired copper thickness in an electrolytic copper plating tank. When the correct thickness of copper is achieved we then plate a thin Tin etch resist layer over the exposed copper and remove the photo resist.  The boards are then etched using a differential etching solution, which removes all the exposed copper areas and leaves behind the final copper track and plated through holes. 

For the uninitiated the best way to visualize the construction technique of a Multi layer board is to picture a series of extra thin double sided boards pressed and glued together and then passed through the Std PTH processes.