The flow of charge through any material encounters an opposing force similar in many respects to mechanical friction. This opposition, due to the collisions between electrons and between electrons and other atoms in the material, which converts electrical energy into another form of energy such as heat, is called the resistance of the material. Resistance is measured in Ohms (Ω), the capital Greek letter omega. The circuit symbol for resistance appears in figure below with the graphic abbreviation for resistance (R).
Most resistors have four coloured band. The first three bands determine the resistance, the fourth band indicates its tolerance. Tolerance is how much the resistance varies drom its specified value, given as percentage : e.g 5% tolerance is indicated by a gold fourth band. It means that a 100 Ohm resistor could actually be between 95-105 Ohm.
E.g., the grey-red-brown-gold resistor is 820 Ohm Resistor.
There are also 5 band resistors. The first four bands determine resistance and the fifth indicates the tolerance generally these are a 1% tolerance resistor, much more accurate than 5% resistor. To determine the resistance, the first three rings are the significant value, the fourth ring is the number of zeros following or multiplier. Eg. 1kOhm is brown-black-black-brown-brown
Resource : My lecture note
Introductuory Circuit Analysis by Boylestad
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