What is an LED?

An LED is a Light Emitting Diode.

A diode is a semi-conductor (an electronic device made from two different metals, which transmit electricity in barely-comprehensible ways) which allows current to flow in one direction only.

A Light Emitting Diode gives off light as a side-effect of this process.

The colour of the light emitted depends on the impurities the electrodes of the LED have been impregnated with.

LEDs have no internal resistance, so must always be wired up in series with a resistor to keep them from burning out... instantly.

LEDs usually come encased in an acrylic capsule, which protects the electronics and acts as a lens.

LEDs generally have 2 electrodes, or 'legs'.
They are called the anode and the cathode.
The anode is connected to the positive side of the power supply and the cathode is connected to the negative.
The anode is the long leg and the cathode is the short leg.
It doesn't harm the LED to be connected the wrong way around, it just won't work.