The nixie tube is a gas discharge vacuum tube, used as a display for numeric and other symbols. The term 'nixie' is actually a trademark of the Burroughs Corporation that has now become a generic name. It was widely used as test equipment display's in the 1950's and early 1960's, as well as calculators, measurement readouts.
In recent years there has been a groundswell of retro electronic builders using these as digital clocks.
Nixies disply elements operate typically with approx 1mA of current from a 170v DC supply, much in the same approach as basic neon lights.