A long put is the purchase of a put option.
The Max Loss is limited to the net premium paid for the option.
The Max Gain is uncapped as the market falls but limited to the strike price minus the stock price as the stock cannot trade lower than zero.
When to use: When you are bearish on market direction and bullish on market volatility.
Like the long call a long put is a nice simple way to take a position on market direction without risking everything. Except with a put option you want the market to decrease in value.
Buying put options is a fantastic way to profit from a down turning market without shorting stock. Even though both methods will make money if the market sells off, buying put options can do this with limited risk.