Resistors can be embedded in PCBs either by employing carbon screen printing or through the use of special material layers that are etched, embedded and connected inside a multilayer pcb as per description further down.
Resistors in PCB´s formed using special high resistance material:
Ohmega-Ply is one of these materials
Ohmega-Ply basic tolerances depend on pcb production equipment and process control with typical tolerance values around +/- 10%. But when resistance layers are produced with larger than required resistances followed by computerized laser trimming equipment tolerances better than 1% regardless of the material used can be achieved at low costs.
More information here http://www.ohmega.com/technology/