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A resistor is a two-terminal electrical component used in electronic circuits to limit the flow of an electric current. Everyone who has dealt with electric circuits knows about resistors. I'll just start with a brief introduction to their lesser-known physical meaning. In general, if the current flow i through an object inserted in a circuit is directly proportional to the voltage V across it, the ratio of V divided by i will be constant. This constant ratio is called resistance: R=V/i. It can be measured by a device called ohmmeter. In an ideal resistor R does not depend on the current over a considerable range of conditions. If by definition R=V/i, then i=V/R. This relationship is referred to as Ohm's law.
Since this relationship seems to be derived from the definition of R, and since it is true only for materials that can be characterized by a constant resistance, it sounds like a circular reasoning. One approach to the Ohm's law is to understand it as a statement that there are substances [including solids and liquids] that display this kind of behavior, i.e. in which V/i=constant at least within certain range of currents. Such substances are called electrical conductors. If we know that an object is a conductor and we know the values of any two of the three quantities in the Ohm's Law, we can calculate the third. Sometimes the term resistance is also used for devices with a non-linear V-I curve. In this case it refers to V/i ratio at a given operating point. We can also define a dynamic impedance as derivative of the voltage with respect to the current. Real physical resistors are characterized by a number of parameters, such as their nominal resistance, tolerance, maximum power they can dissipate without failure, maximum working voltage, temperature coefficient, noise, and parasitic inductance. The SI unit of electrical resistance is the ohm, symbol Ω. Its reciprocal quantity is conductance measured in siemens. Electronic industry often uses a special coding system of colored stripes to indicate the component's value and tolerance (see resistor color chart). These markings were originally published by Electronic Industries Alliance as EIA-RS-279. Currently they are specified by IEC 60062 Ed. 5.0. This graphical calculator works both ways- you can get resistances from color codes or generate the colour bands from ohm values (5%, 10% and 20% tolerance).
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