LED (Light Emitting Diode
LEDs are very efficience lights. LED is an acronym from Light Emitting Diode. A diode is a circuit element that only allows current to flow in one direction. Therefore, it is very important that you put your LED into the circuit with the +ve lead connected to the +ve side of your battery or DC power supply and the -ve lead to the negative.
Note that the longer side is POSITIVE and the flattened side of the light is the NEGATIVE. Note also the correct symbol for an LED in a circuit diagram.