You need:
1 led 5mm
3 pieces of cable
1 resistor 390Ω /0,5W
1 battery 4.5 Volt 
A 9 Volt battery can be used. In this case resistor 820Ω /0,5W should be used.
AA or LR6, or any other cylindrical battery 1,5V can't be used; in this case led propably will not work.

LED (from the initials of the words Light Emitting Diode)
is a semiconductor more info
diode more info inside a bulb made of plastic.
A diode consists of two pieces of silicon each one of which has different infinitesimal impurities from other materials like indium, arsenic, gallium etc.
LED has two terminals the anode (+)
and the cathode (-).more infoIn all diodes electric current can only pass from the one direction, while no electric current passes from the opposite direction. When electric current passes through a LED diode light is emitted. 
An electric source is always connected to a LED in series with a resistor. The resistor limits the electric current and protects the LED.

In order to create a simple circuit with a LED:  the negative pole of a battery is connected to the cathode of a LED, then the resistor (the resistor has no polarity and no matter how the terminals are connected it will operate) and finally the positive pole of the battery. In this case the LED is biased correctly and emits light.

If the LED is connected the other way round that means the anode to the positive pole of the battery, (reversed bias) no electric current passes, and therefore no light is emitted. If the voltage of the battery is higher than 5V the LED might be damaged.
Each LED emits light in a certain frequency (colour). The frequency is determined by its construction. Different LEDs emit different colours (e.g. red, green, blue, orange, yellow, white).
LEDs are used in electronic devices: They form the numbers in digital watches, transmit information from tele-controls, show if a device is on etc.
During the last few years, high power LEDs (2-300W) has been constructed.  They are used to light up homes or street lamps. They are considered more environmental friendly and have longer life span even from the fluorescent lamps.
more info
more info
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© 2003, Despina M. Garyfallidou Athens-Greece