It’s a property of any material to resist flow of current through it. According to this property materials are classified mainly into two parts. One is conductor another is insulator.
In conductors resistant is very less but in insulator resistance is very high.
In circuits resistance is mainly used for controlling the flow of current. They are also classified as under their construction and uses. Example: Carbon, Wire wound, ballast, PTC, NTC, varistor, LDR etc.