A resistor is a little package of resistance: wire it into a circuit and you reduce the current by a precise amount. Take metal film resistor for instance, It is a short, worm-like component with colored stripes on the side. It has two connections, one on either side, so you can hook it into a circuit.

 

What's going on inside a resistor? If you break one open, and scratch off the outer coating of insulating paint, you might see an insulating ceramic rod running through the middle with copper wire wrapped around the outside. A resistor like this is described as wire-wound. 
 

 

 

The number of copper turns controls the resistance very precisely: the more copper turns, and the thinner the copper, the higher the resistance. In smaller-value resistors, designed for lower-power circuits, the copper winding is replaced by a spiral pattern of carbon. Resistors like this are much cheaper to make and are called carbon-film. Generally, wire-wound resistors are more precise and more stable at higher operating temperatures.
 

 

How does the size of a resistor affect its resistance? The length, cross-sectional area, and stuff inside affect resistors' resistance. If you make a wire thinner or longer, it's harder for electrons to wiggle through it. And, as we've already seen, it's harder for electricity to flow through some materials (insulators) than others (conductors).

 

 

 Although Georg Ohm is best known for relating voltage, current, and resistance, he also researched the relationship between resistance and the size and type of material from which a resistor is made. That led him to another important equation: R = ρ × L / S.

 

In simple words, the resistance (R) of a material increases as its length increases (so longer wires offer more resistance) and increases as its area decreases (thinner wires offer more resistance). The resistance is also related to the type of material from which a resistor is made.