Resistors.

http://www.williamson-labs.com/images/resistor-color-code-all.gif

THEORY:
A resistor is a component used in a electronic circuits to resist and regulate the    current/voltage so that other component in the circuit e.g L.E.D (0.02A, 1.8v) get voltage/current that would not exceed its max capability, if there were no resistor in the circuit and the power source is 5v, when circuit is turned on the voltage/current would flow through the L.E.D resulting with L.E.D being blown.

With a series circuit, if there are two resistors end to end that are the same value and power supply is 12v, each resistor would use up 6v but if one resistor is a higher resistance then it would use more of the available voltage and the rest would be used by the other resistor meaning voltage is proportional to the resistance in the circuit.
Ampere in this circuit (series) throughout is always the same from start to finish.

With a parallel circuit, if there are two resistors that were side by side and equal value, then like in a series voltage will need to be used up before returning to earth. When in parallel, amperes is proportional to the resistance meaning if a resistor was of a different value then which ever resistance is higher, that is the resistor that will use most of the ampere in the circuit showing that when resistors are in series resistance is high
and when in parallel, resistance will be low.

TEST:
These were a couple resistors that were tested:
R1 Colour bands:

Red(2), Violet(7), Yellow(4), gold(5% tolerance)
R2 Colour bands:
Brown(1), green(5), red(2), gold(5% tolerance)

Using the colour chart, R1, R2 is calculated as so:

2, 7, 0000, 5% = 270,000 ohms / 270 k.ohms with a tolerance of  +- 5%.(+ 275 k.ohms - 265k.ohms)

1, 5, 00, 5% = 1,500 ohms / 1.5 k.ohms with a tolerance of +- 5%.(+ 6.5 k.ohms /  - 3.5k.ohms)

With these calculations it was certain that R1 was able to handle up to 275,000 and to as low as 265,000 ohms and R2 was up to 6,500 ohms and as low as 0.03 ohms. This shows that these types of resistors are not made to an exact specification or value but have a of 5% either adding to decreasing it.

REFLECTIONS :

Resistors play an important part in a circuit because if there were none and the power supply is 12v, components that require a little amount of voltage or ampere would either heat up, may melt or just blow up. Resistors are a necessity for a safe operation of a circuit.