Resistors can be embedded in PCBs either by use of special layers laminated, etched, connected and optionally buried in a multilayer PCBs, or by employing carbon screen printing as shown further down that also can be buried.
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/
The OhmegaPly RCM is available in a number of standards sheet resistivities expressed in ohms/Sq, 25 ohms/Sq, 50 ohms/Sq, 100 ohms/Sq and 250 ohms/Sq
Formula: Resistance (in ohms) = (Length/Width) x (Sheet Resistivity)
Sheet Resistivity = 100 ohms/Sq
Trace length = 2 mm (80 mil)
Trace width = 0.4 mm (16 mil)
Resistance: 100 x (2/0.4) = 500 ohm = 100 x (80/16) x 500