Resistor, by definition, is a component of an electrical circuit that "resists" the flow of electrical current, or implements electrical resistance. What does it looks like? Well, normally a reisistor has two terminal and is linear. To know better how resistor work, we need to know the Ohm's Law first, which states that the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them. Since current and voltage are directly proportional, the ratio between volatage (across the terminals of the resistor, specifically) and current (through the resistor here) is what we call resistance. After understanding the concept, we need to know how are resistors made. Generally, some resistors have a fix amount of electrical resistance, while some have a variable resistance, and that is why we call them variable resistors. Resistors can be made up of carbon film, oxide film, metal foil and many other types of materials. The most common ones we see in computer motherboards has color codes on them. And here is how you can read them: The first color band on the left represents the first significant figure of the resistance value, the second band is the second significant figure, and the third one is the decimal multiplier, while the fourth one represents the tolerance of value in percent. Sometimes the fourth one does not exist, and that means the tolerance is +/- 20%.