Also known as a vacuum tube, electron tube, or valve, it is an early electronic device in the form of a glass or metal tube with no air inside (so the elements will not burn up in the oxygen). They were used as switches and amplifiers, and still appear in amplifiers and related audio circuits today. When pushed, they are known to distort waveforms asymmetrically, which generates even harmonics (often considered warmer vs. harsh) in addition to the odd harmonics generated by overdriving transistor or IC-based circuitry.
The most basic tube has two elements: a cathode (also known as the emitter), and an anode (also known as the plate). The cathode is connected either to ground or a negative voltage; the plate is connected to a high positive voltage. As the cathode is heated, electrons jump from the cathode to the plate.
In a basic triode tube, a control grid sits between the cathode and plate. A voltage applied here either encourages or inhibits the flow of electrons from cathode to plate. In this way, a small voltage (such as an audio signal) applied to the control grid results in a much larger change in voltage at the plate, causing that input signal to be amplified.
A pentode tube has two additional elements: a screen grid and a suppressor grid. In a typical design, the screen grid amplifies the signal further, while the suppressor grid blocks the “secondary emission” of electrons bouncing off the plate and returns them to the cathode, attenuating the output.