What's in a resistor?
Carbon resistors have a core of (surprise) carbon, which conducts poorly.
Resistance varies with temperature, however, and it's hard to manufacture
these with resistances accurate to better than 5%.
Metal film resisitors have a thin metal film on an insulating core,
surrounded by more insulation. This one is accurate to 1%.
High power or low resistance resistors are often a coil of resistive