The commutation cell is the basic structure in power electronics. It is composed of an electronic switch (today a high-power semiconductor, not a mechanical switch) and a diode. It was traditionally referred to as a chopper, but since switching power supplies became a major form of power conversion, this new term has become more popular.
The purpose of the commutation cell is to "chop" DC power into square wave alternating current. This is done so that an inductor and a capacitor can be used in an LC circuit to change the voltage. This is in theory a lossless process, and in practice efficiencies above 80-90% are routinely achieved. The output is then usually run through a filter to produce clean DC power. By controlling the on and off times (the duty cycle) of the switch in the commutation cell, the output voltage can be regulated.
This basic principle is the core of most modern power supplies, from tiny DC-DC converters in portable devices to huge switching stations for high voltage DC power transmission.
Read more about Commutation Cell: Connection of Two Power Elements, The Structure of A Commutation Cell, The Commutation Cell in Converters
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