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 wire.