Electron tube (thermionic tube) is an electronic component used to amplify (tweak) the electricity current. Electrons are emitted through a heated negative electrode (cathode) and are absorbed by the positive electrode that is usually in form of a metal plate.
Electron tubes look like normal tungsten light bulbs. The biggest difference between them is that there is no direct contact between electrodes in thermionic tubes.
Electron tubes can amplify, rectify (converting alternating current to direct current) and control currents, based on type of the tube.
Electron tubes have been largely replaced with transistors in electronics and IT industry.