All vacuum tubes consist essentially of an electrically heated resistance which emits electrons, and various other electrodes which control the flow and destination of these electrons. This must take place in a vacuum.
Some tubes fill one function only, as in the early days, for example : diode 1A3, triode 27, hexode VP2B etc. With time, tubes were developped which filled two functions, for example : triode pentode ECL80/6AB8, double triode 12AX7/ECC83, and later three functions, for example : double diode triode 6R7, double triode pentode 23Z9. Still later four functions, for example : quadruple diode 6AN6.
We also have in inventory tubes with other functions.
Ballast tubes (for stabilising voltages), Nixie tubes (showing visible digits for counting purposes), Photo tubes (which react to light), and Relay tubes (which operate as switches).