There are two types of nightlife in Istanbul: Turkish and Western. Turkish nightlife involves turku bars where they play turkish folk music, live performance bars for famous Turkish singer and belly dancing shows.
When it comes to Western style you have everything you can find in a major Western capital, bars, discos, nightclubs, clubs, revues.
Taksim area is the place for nightlife, Istiklal street and then then there are clubs and bars along the Bosphorus strip.
Needless to say there are other nighlife venues outside Taksim and Bosphorus strip but they are not as concentrated. Taksim Beyoglu area by far offers the most alterntives. Vista are great at some rooftop bars and restaurants whether they are hotel roofs or private apartment building rooftops. Some of them in the Odakule area, midway between Tunel and Taksim offers some of the amazing views over the Bosphorus and the Old Town.
In recent years Kadikoy on the Asian side has become a lively nightlife hub with its own bar streets and meyhanes, offering a more local alternative to the Taksim scene. Bosphorus waterfront clubs in Ortakoy and Bebek are popular in summer months with outdoor terraces.