LED contain gases within acrylic casings. The gas is ignited by the cathode-anode when powered producing the color (wave length) associated with the gas.
LED require constant current or voltage drivers since (household) or typical wall pack's fluctuate in output, taking chances in frying the LED.
LED require a ballast resistor, of sorts because they require very little energy to glow.
Use E=IR to calculate the resistor needed when working with LED, depending on the voltage of your power source.
Discrete white LED are typcially rated 3.6vdc at 30mA. Drive them at 20-24mA instead. This assures their longevity of about 11.4 years even though 30mA yields brighter light.
Keep in mind that a 12v car battery yields more than 12 volts.
LED degrade to 70% after about 5 years, typically, though not dramatically noticable to the human eye.
Here's a more detailed look: