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 and other instrumentation.
In recent years there has been a groundswell of retro electronic builders using these as digital clocks, wrist watches, retro CD players and even punked up AM radio receivers.
Nixies disply elements operate typically with approx 1mA of current from a 170v DC supply, much in the same approach as basic neon lights. They range in size from miniature (approx 8mm) to extra large (100mm) figure sizes.
A recent retro project of a digital clock is shown below, repurposing an otherwise obsolete unit destined for the re-cycle yard.