The standard time zone in Turkey is UTC/GMT +2 hours. During the Daylight Saving Time (DST), the difference is +1 hour. All provinces in Turkey, including Istanbul, are within the same time zone therefore the clock in Turkey is always the same throughout the country. The actual time in Istanbul (and in Turkey, of course) is:
GMT: The GMT stands for "Greenwhich Mean Time". Greenwich is the place from where all time zones are measured. GMT is sometimes called Greenwich Meridian Time because it is measured from the Greenwich Meridian Line at the Royal Observatory in Greenwich, England. The Greenwich Meridian (Prime Meridian or Longitude Zero degrees as Longitude 0° 0' 0") marks the starting point of every time zone in the World. The GMT is the average time that the Earth takes to rotate from noon-to-noon, it's the basis of every world time zone.
UTC: The UTC stands for "Universal Time Coordinated". It is known as the "atomic time" which lately has replaced the GMT becoming the international time standard. The UTC is equivalent to mean solar time at the prime meridian (0° longitude), formerly expressed in GMT. It is also equivalent to Zulu Time. UTC is calculated at the Bureau International des Poids et Mesures (BIPM) in France.
Zulu Time: The Zulu is used especially by the military and aviation and marks precision time like the GMT or UTC.
DST: The DST stands for "Daylight Saving Time", known also as "summer time". It is the aggreement of energy saving by advancing clocks so that afternoons have more daylight and mornings have less, forwarding one hour at the start of Spring and are adjusted backward in Autumn. Turkey is one of the countries that applies DST like many others in the northern hemisphere (Europe, Russia, USA, Canada, and so on). Turkey at the moment follows Eastern European Summer Time, but in 2011 the daylight saving time will be abolished and we will adopt a new time zone (GMT + 2:30 hours) throughout the year (with no Summer Time).