The Half Wave Rectifier functions using a single diode. It rectifies only half of the sine wave. 
During the positive half cycle, the diode D1 is in forward bias and current flows to the load. In the negative half cycle, when the voltage is applied in the opposite direction, the diode is in reverse bias and no current flows.
The Half wave rectifier is a simple device which requires only one diode to be put in series with the load.
The half wave rectifier has a low power output.
Besides, the ripple content of the rectified DC supply is very high. This can damage the loads.
It has very few applications and is only used in emergencies as a temporary measure.