What are resistors used for?
Resistors are used to reduce voltage or current in a circuit. Of course a resistor will do both since they are all related through
Other parts of a circuit may require limiting current. For instance you have a capacitor that is fully charged and you want to discharge it but limit the current. Again, you would put a resistor in series to do this.
A resistor that provides an variable amount of resistance is called a potentiometer. These are commonly attached to knobs. For example a volume control on a stereo; the "pot" is connected to the base of a transistor to control the amount of amplification to a speaker. Another example is controlling the amount of power going to an electric motor.
There are other uses too, but these are the basic ones.