Although Turkey is situated in a geographical location where climatic conditions are quite temperate, the diverse nature of the landscape, and the existence in particular of the mountains that run parallel to the coasts, results in significant differences in climatic conditions from one region to the other.
Istanbul is part of Europe and the weather reflects that. You'll find a wet cold winter (3-8ºC) slowly turning into a damp warm spring and a mild (18–28ºC) summer that doesn't hang around any longer than it feels is necessary.
The Aegean and Mediterranean coasts have a typical Mediterranean climate with hot summers (21-33ºC) and mild winters (4-12ºC).
Cappadocia shares the climate of much of inland Turkey. Summers can be very hot and your altitude is directly proportional to the amount of snow you get in the winter.