A diode is a device that conducts the current in just one direction: the direction of the arrow in the diode symbol, which looks like this:

The most important parameters of a diode are: maximum forward current, forward voltage, maximum power dissipation and reverse voltage.

The forward current is the current flow in the direction of the arrow of the diode symbol. This current causes a voltage across the diode: the forward voltage drop.

Each diode has a certain minimum voltage drop, called the knee voltage. The diode will not conduct when the voltage across it is less than the knee voltage. The knee voltage of a generic silicon diode is about 0.6V.

The power dissipation of a diode is the forward current multiplied by the forward voltage drop.

The reverse voltage is the voltage across a diode when it is reverse biased.

If you want to know how a diode works internally, you'll have to take a peek inside.