American physicist Lee De Forest invented the triode (three-electrode vacuum tube) in 1907, which he named the Audion. The Audion triode built upon John Fleming's discovery of the diode just a few years before. In a diode, electric current flow is either on or off, but the Audion triode was a critical leap forward because current can be all-the-way-on, all-the-way-off, or anything in between. The triode is able to do this by using a tiny wire grid placed between the cathode and the anode to form the third electrode, simply called the grid, which controls the flow of electrons.
The tutorial above shows the inner workings of an Audion triode. An electric current heats the filament (cathode) until it emits electrons (depicted here as yellow particles) that are attracted toward the positively charged plate (anode). The electrons are slowed down before getting to the plate by the negatively charged grid. Use the grid voltage slider to vary the strength of the negative charge of the grid. Higher voltage blocks more electrons from reaching the plate, resulting in less amplification. A weaker grid voltage increases amplification by allowing more electrons to reach the plate and flow up the attached wire.
Originally, the Audion triode was used to detect and amplify signals in early radios, but its sensitivity and versatility made it a critical component in amplifiers for voice and music. Many improvements were made to the Audion triode, adding grids to create tetrodes and pentodes, but vacuum tubes have all but been replaced by semiconductor transistors in most electronics. An exception is guitar and musical amplifiers where the quality of the "tube sound" is still revered.