The diode is two terminal non linear device whose I-V characteristic exhibits non-linear behavior which is also polarity dependent.
The basic circuit symbol of the diode is shown on Figure 1. The behavior of the diode depends on the relative polarity of its terminals. The figure also shows conventional voltage polarity across the diode terminals and the current direction through the diode.
Depending on the polarity of the voltage Vd the diode is said to be:
Forward Biased, (Vd >0), Anode voltage is greater than the Cathode voltage, or
Reverse Biased, (Vd<0), Cathode voltage is greater than the Anode voltage
Practical Diode Model
Diode Circuit Analysis