They are licensed to the respective borough and sell either one of the following three items or a combination:
Simit (tasty sesame bread ring) and other pastry/bread type snacks
Grilled corn and roasted chestnuts
There are also plenty of non-licensed carts with a variety of products and one that caught our eye was sahlep.
It's typically drunk in the colder months and goes back to the Ottoman Empire. It is creamy and based on a mix of different spices and served hot with a generous sprinkle of cinnamon on top.
Another Turkish classic I need to mention is Boza Vefa and if you are in Istanbul you have to try it. The one we went to is a popular tourist attraction in it's won right: Vefa Bozacisi.
It's a cold drink made from a fermented bulgur and it is served with cinnamon and roasted chickpeas (Leblebi in Turkish, though you have to get yours from the shop over the road). It is rich in carbohydrates and vitamins.