Dolmus and Minibus -- Getting Around in Istanbul ?

A dolmus (which means 'full') is basically a shared taxi that sets off once every seat is taken. Dolmus run fixed routes (starting points and final destinations are displayed in the front window) but with no set stops. Passengers flag the driver down to get on (if there's room) and holler out to be let off (Inecek var!). For local journeys, there's one fixed fare. Ask a fellow passenger how much it is or just watch what everyone else is paying. Dolmus run later than buses, often as late as 2am.

Minibuses are more crowded than dolmus, and less frequent. Minibus fares are lower, but chances are you'll make your journey standing while being blasted by tinny Turkish pop. Pay and get on/off as you would a dolmus. The main routes are from Besiktas to the upper Bosphorus districts.

Source: http://www.timeout.com/istanbul/features/308/travel-information-getting-around-istanbul