What is "rectification", why is it so useful, and what does a rectifier have to do with it?

A rectifier is a piece of electronic kit that converts A.C. to D.C. This process is known as rectification.

Rectifiers are often found as components of D.C. power supplies (many electronic circuits require a steady D.C. current e.g. radio, television and computer equipment).

The opposite process, of converting D.C. to A.C., requires more complex circuitry in the form of an inverter. Different types of inverters produce different "quality" waves ranging from square waveforms, sine waves to modified sine waves.