Twilight is that time of day, especially in the evening, when the sun is below the horizon but continues to supply some light to the sky. Is is a beautiful time of day for photography. The key is to balance exposure so the rich deep color of sky is neither too dark nor too light, but the other elements of the scene are also exposed well. This works well in a cityscape where the artificial lights of the city will more or less balance with the sky. Of course this does not usually work well with an overcast sky. Here is an example from Albemarle, North Carolina on a clear February evening, taken with a Sony NEX5N and a Voigtlander 15mm lens.