Resistors are used as current limiters. Usually they are use in series or in parallel. These range from 1 ohm to 10mega ohm. They are used in battery chargers to limit current, amplifiers to bias current, regulators to set the reference values, etc.
When we dont have required resistor values ,we can use the values we have to get the required value through series and parallel combinations. Resistor value adds in series and divides in parallel. If R1 and R2 are in series then the total value is R1+R2. Similarly if R1 and R2 are in parallel then the total resistance is given as R1R2/(R1+R2).
When we dont have the required power ratings of the resistors we can also make parallel and series combinations to increase the power rating and in both cases the power increases. For example if we need 1watt 1kohn resistance then we can combine two half watt 2kohm resistances in parallel or two 500ohm resistance in series and we get the required resistance in both the cases.
Apart from constant resistors there are also variable resistors called potentiometers. In these the value of resistance can be varied by turning the knob. These are also available in various power levels but the cost of higher power potentiometers is more than 20times the cost of the regular(1/10th watt) pots. Apart from normal single turn potentiometers there are also multi turn pots available to have more precision in the resistance values.
