What are commodity exchanges?

That's the equivalent of a stock market for "commodities" usually stuff like oil, wood, grain, whatever. They trade "Futures" which are future delivery contracts that usually promise delivery in 3-4 months. So basically when you buy 1mln oil futures (let's ume 1 barrel=1 future) if you don't sell it within the period when you buy and the delivery time, they're going to show up with a million barrels of oil at your house.