[i_am_proteus]

Vacuum tubes (thermionic valves) work by heating a tungsten filament-- the cathode-- until the filament is hot enough to emit electrons, which are then attracted to the positively-charged plate (anode), traveling through the vacuum (and thus flowing current). Current only flows in one direction in this setup, which has two electrodes, leading to the moniker "diode." Adding a third electrode in between the cathode and anode-- called the "grid" based on its physical shape-- allowed a vacuum tube (now a 'triode') to use a small signal to control a large one. An amplifier!

The key to the function is the heating of the cathode filament. Vacuum tube designs that use a separate heating current (which is, today, most of them) do not conduct if the heating current is not applied. "Cold cathode" type tubes are not vacuum tubes-- they are typically filled with a low-pressure gas.

[Zardoz84]

a vacum diode have characteristics more similar to a ideal diode that a semiconductor diode.