The original three-element device was patented in 1908 by Lee De Forest who developed it from his original two-element 1906 Audion. The Audion did incorporate, in a crude form, the key principle of allowing amplification. However it was not until around 1912 that other researchers, while attempting to improve the service life of the audion, stumbled on the principle of the true vacuum tube. The name triode appeared later, when it became necessary to distinguish it from other generic kinds of vacuum tubes with more or less elements (eg diodes, tetrodes, pentodes etc.). The original Audion tubes were not vacuum tubes however, as they deliberately contained some gas at low pressure. The name triode is only applied to vacuum tubes.
The principle of its operation is that, as with a thermionic diode, the heated filament causes a flow of electrons that are attracted to the plate and create a current. Applying a negative charge to the control grid will tend to repel some of the (also negatively charged) electrons back towards the filament: the larger the charge on the grid, the smaller the current to the plate. If an AC signal is superimposed on the DC bias of the grid, an amplified version of the AC signal appears in the plate circuit.
Although triodes are now largely obsolete in consumer electronics, having been replaced by the transistor, triodes continue to be used in certain high-end and professional audio applications, as well as in microphone preamplifiers and electric guitar amplifiers.
Some guitarists routinely drive their amplifiers to the point of saturation, in order to produce a desired distortion tone. Many people prefer the sound of triodes in such an application, since the distortion of a tube amplifier, which has a "soft" saturation characteristic, can be more pleasing to the ear than that of a typical solid-state amplifier, which is linear up to the limits of its supply voltage and then clips abruptly. However, this typically only applied to the power stage of a tube amplifier.