A put spread is a position comprised of long puts and short puts.

A debit put vertical is a bearish options strategy that buys one put and sells another put at a lower strike on the same date. This is sometimes called a bear put spread. 

Opening a debit put vertical can be a good strategy with a high payout if testing a Call Wall or Volatility Trigger™ from underneath, or if falling under a Put Wall.

A debit put vertical is mostly a directional play. When it is right about the price dropping sharply, then it usually has a bonus from a sudden increase in IV (implied volatility). As another luxury, this strategy reliably has a [skew] edge when bought on index products, and on most (but not all) stocks.

A credit put vertical is short a put and then long a put at a lower strike. 

Using a credit put vertical can be a high-accuracy strategy if opening it above well established support, such as when testing a Put Wall from above for a potential bounce. 

A trader would want to use a credit put vertical if expecting any outcome other than a sharp decline in the underlying security’s price. In other words, this is more of a “not bearish” trade than a bullish trade. 

The max loss is limited to the difference between the put strikes, minus the net premium collected upfront. Max profit is the net premium collected upfront.