The aim of this paper is to investigate the glow discharge in its diffuse and filamentary regime, and to study its transition to arc in argon at 100 mbar. The structure of the discharge is observed in correlation with its electrical signals in both static and in dynamic modes. Glow discharges were identified with a positive column that can be fully diffuse, fully filamentary, or a mix of the two with the filamentary part of the column always attached to the anode. Spontaneous transitions between glow discharges and arcs have also been observed and their dynamics are studied. Using high-speed imaging, the transition between glow and arc discharge was identified as the occurrence of two distinct phenomena: a propagation mechanism in the positive column and a constriction of the cathode root. An evaluation of the duration of each phenomenon as a function of electrode gap and current intensity was obtained. The duration of the constriction of the cathode root is found to be on the order of tens of nanoseconds and the duration of the propagation mechanism in the range 50-550 ns.