Explain the difference between a put option and a short position in a futures contract?

Well, the first difference is the root difference between a futures contract and an option contract: in a futures contract you MUST complete the sale at the end of the contract (if you didn't buy it back before the settlement date) but in an option you CAN.

Once we're past that, the short position in a futures contract--the person who has the item the contract is derived from, such as a thousand bushels of wheat--is the same as the buyer of a put. Both of them have the thing now, and will transfer title to it after settlement or exercise.