A rectifier is any device which forces alternating current through it to flow in one direction only. So, this process is called rectification. Normally, two terminal devices called diodes are used for this purpose, but at high power levels where a measure of magnitude control is additionally desired three terminal devices like thyristors, triacs, and FETs may also be used. (Not all diode types are used for rectification purposes. Varactors and zeners are examples of diodes for other purposes.
When current is flowing through a rectifier in the forward direction, (for a diode this is from the anode to cathode) the voltage drop in the rectifier is near zero (about 0.7 v in a silicon diode). The entire applied voltage is dropped across it in case a reverse voltage is applied. The highest reverse voltage a diode or a rectifier can withstand is called its reverse breakdown voltage. The maximum current a rectifier can conduct in the forward direction without being damaged is called the Maximum Forward current. The ones with forward currents from milliamps to hundreds of amperes and breakdown voltages from tens of volts to thousands of volts are available.
The simplest rectifier circuit is the half-wave rectifier employing only one diode in series. A half wave rectifier allows only one half of a sine wave to pass and blocks the other half. The output needs filtering or smoothing. The average value of DC present in the output of the half rectified sine wave is 1/π of the peak value of the input (ignoring the diode drop of approximately 0.7 v).
A full wave rectifier can be made in more than one ways, you can see details here: https://www.electronics-tutorials.ws/diode/diode_6.html. The most compact is the bridge rectifier using four diodes in a specific configuration called a bridge. The bridge does not provide isolation between the input and the output. It is almost like a single diode in series with circuit, except that that the forward drop is from two diodes, and both halves of the sine wave are conducted but in the same direction. Thus, the output voltage is double of that from a half wave rectifier, or 2/π of the peak voltage.
Where isolation between the input and output is required, a transformer with a center tapped secondary can be used with two diodes. And, this arrangement is bulky and is avoided unless otherwise necessary.
Rectifiers may be power rectifiers or signal rectifiers. Signal rectifiers are low level rectifiers used in communications circuits. Signal rectifiers are normally called detectors. For them, the reverse voltage and forward current are not the critical ratings. Rather, the forward resistance and shunt capacitance are important because they together limit the highest frequency of operation. Research is continuously on to raise the upper frequency limit of detector diodes.