Voltage sags are short duration reductions in rms voltage caused by short circuits, overloads and starting of large motors. Voltage sag is much more of a global problem. For proper analysis and mitigation of voltage sag, their characterization is important. The magnitude and durations are main characteristics of voltage sag. In this paper voltage sag and their characteristics have been presented in a comprehensive way. How voltage sags occur, what are their characteristics and impact on equipments behavior for different conditions.