A type of general store which populates the US/Canadian border. "Duty free" refers to the fact that they charge no taxes. How do they get away with this? No taxes apply to goods exported from Canada to the US. Thus you must be leaving the country in order to buy the goods in such a manner; this makes duty frees very popular for US vacationers.
Duty frees carry anything and everything you might think of: candy, snacks, jewelry, medicine, alcohol, even turkey jerky. They will also process tax refunds for you.
The French word for a duty free is hors taxes, so most stores are entitled "Boutique Hors Taxes/Duty Free".
ocelotbob informs me that duty free shops also exist internationally, mostly in airports. I imagine that the same export laws apply there, though I can give no firsthand information about the types of goods or services found therein.