LED stands for Light Emiting Diode
There are no different values of LED but they come in a shapes and sizes.
The diode part of the LED means that to emit light it must be connected
the right way around. It won't brake it if it's connected backwards it
just won't give off any light.
The more negative side of the LED is indicated by the short lead or the
flat edge. The flat edge is most commonly used to determine the LEDs
orientation.