Rectification is the process of converting the alternating voltage and current to into the corresponding direct (DC) quantity. The input is AC where as the output is unidirectional or DC. This device is know as a rectifier.
A step down transformer is used to reduce the ac mains voltage to an adequately small value. The turn ratio of the transformer is adjusted to obtain a stepped down AC voltage. This voltage is converted into a pulsating DC voltage by the rectifier. The type of rectifier are Half wave rectifier, full wave rectifier or bridge rectifier.