Glow Discharge Tube
Electric spark occurs due to dielectric breakdown of the medium (e.g. air) under high electric field or potential difference. This happens when the potential difference is sufficiently high to overcome the ionization energies of the atoms/molecules. The ions and electrons are thus pulled apart and accelerated towards the opposite polarities of the electric field. The speeding charged ions and electrons conduct electric current. Collision ionizations and excitations occur as the energetic electrons impact on the neighboring neutral atoms/molecules (see the Franck-Hertz Experiment). When an excited atom/molecule de-excites and falls to a lower energy state, a photon is emitted. The medium glows, i.e. emitting light, if the photons emitted are within the visible spectrum.
The glow discharge tube makes use of the light emission from the atomic/molecular de-excitations. The following italic text is taken from an Wikipedia article:
The simplest type of glow discharge is a direct-current glow discharge. In its simplest form, it consists of two electrodes in a cell held at low pressure (0.1–10 torr; about 1/10000th to 1/100th of atmospheric pressure). A low pressure is used to increase the mean free path; for a fixed electric field, a longer mean free path allows a charged particle to gain more energy before colliding with another particle. The cell is typically filled with neon, but other gases can also be used. An electric potential of several hundred volts is applied between the two electrodes. A small fraction of the population of atoms within the cell is initially ionized through random processes, such as thermal collisions between atoms or by gamma rays. The positive ions are driven towards the cathode by the electric potential, and the electrons are driven towards the anode by the same potential. The initial population of ions and electrons collides with other atoms, exciting or ionizing them. As long as the potential is maintained, a population of ions and electrons remains.