Value-added tax (VAT; KDV in Turkish) in Turkey is 18%, with a reduced rate of 8% applied to items including textiles, clothes, leather goods, carpets, shoes and books, and a super-reduced rate of 1% applied to items including newspapers. Bills should clearly state the tax included.
If you buy an item costing more than ₺100 + KDV from a shop participating in the national 'Global Refund: Tax Free Shopping' scheme, you are entitled to a KDV refund at your point of departure. The shop will give you a form to complete and present at the airport along with your purchases.
Sadly, this system doesn't always work, so it is best to only pay a price you feel is fair without the prospect of a VAT refund. There have been cases of shops abusing the system and giving the buyer a form in Turkish which actually says the refund has been paid, allowing the shopkeeper to keep the fee.