A rectifier is an electronic device used to convert alternating current (AC), such as ordinary house electricity, into direct current (DC). Without rectifiers, it's likely that things such as Computers, radios, and television sets, which require DC for proper operation, would have to be powered exclusively from batteries - or not powered at all!
Whether vacuum tube or semiconductor, most rectifiers contain two electrodes: an anode and a cathode in the semiconductor version, or plate and cathode in the tube version. These two-terminal devices are known as diodes. Three-terminal devices, such as triodes or bipolar transistors, can be used but are not as suited to rectification as diodes.
In operation, a rectifier acts as an electronic switch and will conduct current in only one direction. Therefore, when an AC voltage is applied to a rectifier, current flows for only one-half of the cycle, during the positive swing of the cycle. In practical use, rectifier diodes are configured in half-wave, full-wave, or bridge circuit configurations.
The half-wave rectifier circuit uses only one diode and is the least efficient configuration. Its major (and some would say only) justification is its low cost and relative simplicity. Much more efficient is the full-wave rectifier circuit. In this arrangement, the outputs of two half-wave rectifiers are combined such that current flows during both halves of the AC cycle. This circuit requires much less filtering of the output DC voltage than the half-wave circuit and provides better regulation of the output.
The bridge rectifier circuit requires four rectifier diodes. These diodes are connected in a ring configuration, such that two diodes conduct on each half of the AC cycle. Thus, it is also a full-wave circuit but does not require a center tap on the secondary of the power transformer (as does the two-diode circuit).
The demodulator in most radios (that part of the circuit that changes the radio signal to an audible signal) is also a rectifier. It operates on a much smaller scale of voltage and current, but is a rectifier nonetheless.
Before the advent of semiconductors, most radio receiving and transmitting equipment required one and sometimes two huge rectifier tubes. Today, in all but high-current or high-voltage applications, vacuum tube rectifiers have been supplanted by semiconductor devices. Semiconductors are more reliable and usually run cooler than tube devices. For their high-end amplifiers, though, audio enthusiasts still require tube rectifiers.
ARRL Headquarters Staff. The Radio Amateur's Handbook. Hartford, Connecticut: The American Radio Relay League, 2001.
Horowitz, Paul and Hill, Winfield. The Art of Electronics. New York: Cambridge University Press, 1985.