A LED (light-emitting diode) is a semiconductor light source. A LED glow when current flows in the conducting direction.
As diode the LED have two circuit points minimum, anode (+) and cathode (-). At the most LEDs, anode and cathode are labelled. The shorter conductor is the cathode at different connection wires. Guideline: "Long is always positive".
Typical parts of a 5mm LED composed of body, anode and cathode.
LEDs aren't thermal spotlights. The emitting light corresponds to a limited spectral range. Until a few years ago it was not possible to produce all colours of the visible light by means of LEDs. There are the blue LEDs only for some years. With the development of blue LEDs, it was at the first time possible to develop LEDs which glow white. For this, blue LEDs are provided with a changing hue layer.
The lighting current is proportional to the operational current which is depending on temperature. The typical solution is to use Constant Current Sources, at most in the form of a series resistor.
Because of the physical qualities the LEDs are subject of a natural ageing, also called degradation. The life time of LEDs depends on the semiconductor materials and the operating conditions. High temperatures drastically reduce the life time of a LED. Often an adequate heat sink is needed for a long life.