Nice explaination @ veeralakshmi
However, i'll give a bit more indepth info how it is created.
If an electromotive force (emf) is applied to a pair of metallic conductors that are separated by a small gap, the flow of current is essentially zero. Now, if we increase EMF progressively to a very high levels say thousands of volts and the gap is viewed in the dark, a faint glow, associated with a tiny current flow, becomes visible. This is the region of the silent electric discharge.
This glow becomes much stronger when EMF is increased further. Eventually, at some critical higher EMF region, the system enters the highly visible arc or spark region. This means system is not in Silent electric discharge anymore.