There are several different technologies. The most common way
hobbyists make them is to purchase board laminated with one or both
sides covered with copper. An etchant resist pattern is created is
made with a resist pen, or by several other techniques, or by buying
boards that are also covered with a photo resist layer. That later
has ot be exposed to UV through a pattern to make some areas soluble.
The board is then treated to an alkaline bath (or some versions of
resist use an organic solvent ot remove part of the resist). Then the
board is bathed in an etchant that dissolves the unprotected parts of
the copper layer. Holes are then drilled through the board and some
people add brass eyelets through the holes to connect the copper on
one side ot the other side.
The processes used commercially are quite a bit more varied and
complicated.
http://www.pwtpcbs.com/makingpcbs/index.html
http://www.kepro.com/howto.htm
http://www.ncws.com/rcrock/makepcb.htm