A short put is simply the sale of a put option.
Risk / Reward
Maximum Loss: Unlimited in a falling market, although in practice is really limited to the strike - zero as a stock cannot trade negative.
Maximum Gain: Limited to the premium received for selling the put option.
When to use: When you are bullish on market direction and bearish on market volatility.
Like the Short Call Option, selling naked puts can be a very risky strategy as your losses can be significant in a falling market.
Although selling puts carries the potential for large losses on the downside they are a great way to position yourself to buy stock when it becomes "cheap". Selling a put option is another way of saying "I would buy this stock for [strike] price if it were to trade there by [expiration] date."
A short put locks in the purchase price of a stock at the strike price. Plus you will keep any premium received as a result of the trade.
For example, say AAPL is trading at $98.25. You want to buy this stock buy think it could come off a bit in the next couple of weeks. You say to yourself "if AAPL sells off to $90 in two weeks I will buy."
At the time of writing this the $90 November put option (Nov 21) is trading at $2.37. You sell the put option and receive $237 for the trade and have now locked in a purchase price of $90 if AAPL trades that low in the 10 or so days until expiration. Plus you get to keep the $237 no matter what.
The risk here is that the stock tanks before the expiration date leaving you with the potential to be exercised and take delivery of the stock at $90 when it, say, is trading at $80 when you are assigned the stock.
If the drop occurs early, and it is significant i.e. at or below the strike, you would want to re-evluate your trade and potentially exit the option position before the losses increase. If the drop in stock value occurs close to the expiration date and is not yet through the strike price, a good exit plan is to put a short stop order on the stock itself. That way you'll be covered on the exercise if it happens while leaving the option position open to capture the remaining time value.